GIFT  OF 


Consulting  Engineer 


ELECTRICAL   ENGINEERING 

AN    ELEMENTARY   TEXT-BOOK 

SUITABLE    FOR 

PERSONS    EMPLOYED    IN    THE    MECHANICAL    AND    ELECTRICAL 
ENGINEERING    TRADES,    FOR* ELEMENTARY    S'lUDINIS 

OF    ELECTRICAL    ENGINEERING,    AND    FOR 

ALL    WHO    WISH    TO    ACQUIRE    A    KNOWLEDGE    OF    THE    CHIEF 
PRINCIPLES    AND    PRACTICE    OF    THE    SUBJECT 

BY 

E.     ROSENBERG 

Chief  Electrical  Engineer  at  Mess*-*.  Korting  Bros.,  Hanover 


TRANSLATED    BY 

W.   W.   HALDANE  GEE       AND        CARL    KINZBRUNNER 

B.Sc.  (Lond.),  A.M.I.E.f-l.  Lecturer  on  Electrical  Engineering  at  the 

Professor  of  Applied  Physics  at  the  Municipal  Municipal  School  of  Technology^ 

School  of  Technology^  Manchester  Manchester 


AUTHORIZED    EDITION 


REVISED   AND    BROUGHT    DOWN    TO    DATE    FOR    THE    AMERICAN    MARKET 

BY 

EDWARD    B.    RAYMOND,  B.S. 

Associate  Member  of  the  American  Institute  of  Electrical  Engineers 
General  Superintendent  Schenectady   Works  of  the  General  Electric  Company 


NEW    YORK. 

JOHN    WILEY    &    SONS 

43   AND    45    EAST   NINETEENTH    STREET 

1907 


Copyiight    1903,  1906 


JOHN    WILEY    &    SONS 


ROBFRT    DRUMMONn.    PRINTFR,    NKW    YOKK 


r* 

Consulting  Engineer  ] 


MASON  STS 


PREFACE 


THIS  book  had  its  origin  in  a  number  of  lectures  which  I  delivered 
two  years  ago  to  the  workmen  and  the  staff  of  a  large  electrical  manu- 
facturing firm.  The  circle  of  readers  for  which  this  book  is  intended 
is,  in  the  first  place,  the  same  as  that  to  which  my  audience  belonged. 
It  should  give  to  workmen  of  electrical  engineering  works  the  knowl- 
edge of  the  operation  of  machines  and  apparatus  with  which  they  are 
concerned.  I  have  endeavoured  to  use  such  language  that  people  who 
have  only  a  general  school  education  should  be  able  to  understand. 
For  this  reason  several  matters  have  been  dealt  with  very  completely 
which,  to  the  mathematically  educated  man,  could  have  been  explained 
in  a  few  lines — such  as,  for  instance,  Ohm's  Law  and  resistance  calcu- 
lations. I  have  in  these  cases  endeavouied  to  explain  the  matter 
first  without  the  help  of  mathematics,  and  then  have  finally,  as  a  key- 
stone— after  having  worked  some  examples — stated  the  formulae  In 
the  part  relating  to  the  "output  of  a  three-phase  current  system," 
in  a  similar  way,  there  are  calculations  and  formulae,  since  it  is 
necessary  for  the  student  to  be  acquainted  with  these.  Generally, 
however,  calculations  and  formulae  are  avoided,  as  in  the  case  of  the 
whole  chapter  about  dynamos.  The  book  will  not  enable  the  reader 
to  calculate  the  parts  and  windings  of  dynamos,  and  he  should  not  even 
think  that  he  is  able  to  do  so.  For  the  elementary  student,  the  wire- 
man,  for  the  engineer  as  well  as  for  the  general  public  who  desire  to 
know  something  about  electrical  engineering,  it  is  quite  sufficient  if 
they  understand  the  working  of  dynamos,  their  faults,  and  the  reason 
and  the  cure  of  the  latter. 

The  book  covers  a  wide  area.  It  comprises,  besides  the  funda- 
mental phenomena  of  the  electric  current,  dynamos  and  motors 
for  continuous,  alternating,  and  three-phase  current,  then  accumu- 

vii 


vm  PREFACE 

la  tors  and  their  apparatus,  measuring  instruments  and  electric  light- 
ing. All  these  things  must  be  known  by  an  electrical  engineer.  It 
was,  of  course,  impossible  to  deal  equally  completely  with  all  these 
themes.  It  is  not  possible  to  describe  all  shapes  of  dynamos,  and 
many  types  of  arc  lamps  and  measuring  instruments.  Only  the  most 
important  types  are  dealt  with,  and  their  working  is  explained. 
Further  detail  is  impossible,  having  regard  to  the  extent  of  the  book, 
and  I  do  not  consider  it  to  be  necessary.  Every  electrical  firm  pub- 
lishes complete  pamphlets  about  their  special  manufactures,  and  in 
extraordinary  cases  the  installer  gets  special  diagrams  of  connections 
and  specification  for  the  plant  he  has  to  erect.  Besides  that,  any- 
body who  understands  the  machines  and  apparatus  described  in  this 
book  will  be  able  to  make  himself  clear  as  to  other  types. 

About  laying  mains  and  about  installation  material  but  little  is 
said  in  this  book.  The  necessary  information  may  be  found  else- 
where. 

Although  this  book  is  chiefly  for  electrical  men  and  for  those  who 
intend  to  become  such,  yet  the  general  public  desiring  to  get  some 
general  information  about  Electrical  Engineering  will  read  the  book 
with  advantage.  I  am  also  in  hope  that  in  some  of  the  chapters 
useful  hints  may  be  found  by  the  educated  electrical  engineer. 

My  object  has  been  a.  practical  one,  and  the  arrangement  of  the 
material  has  been  made  from  a  practical  point  of  view.  I  have,  there- 
fore, departed  in  some  cases  from  the  historical  order;  for  instance,  in 
the  case  of  dynamos.  Again,  facts  have  been  omitted  which  are 
indeed  necessary  for  the  scientific,  but  not  for  the  practical  treatment 
of  the  matter;  for  example,  in  describing  accumulators  and  the 
induction  effects  of  alternating  currents. 

E.  ROSENBERG. 

HANOVER, 

January,  1902. 


TRANSLATORS'   PREFACE 

WE  have  undertaken  the  translation  of  Herr  Rosenberg's  "Elektrische 
Starkstrom-Technik"  with  the  idea  that  the  book  will  be  distinctly 
helpful  to  less  advanced  students  of  electrical  engineering  in  the 
English-speaking  countries. 

It  is  the  work  of  an  electrical  engineer,  and  is  writen  from  an 
engineering  standpoint.  Its  origin  is  explained  in  the  Author's 
Preface,  and  the  opinion  of  German  critics  is  seen  to  be  favourable 
from  the  Press  Notices  which  are  given  at  the  end  of  the  volume. 

In  one  way  the  work  is  different  to  others  of  an  elementary  kind 
— in  the  space  that  is  given  to  alternating  current  electrical  engineer- 
ing. This  subject  is  usually  practically  ignored,  or  is  treated  so 
mathematically  that  it  is  quite  beyond  the  powers  of  most  readers. 
We  feel  sure  that  here  the  work  will  not  only  be  of  value  for  its  clear 
explanation  of  principles,  but  also  for  the  useful  practical  hints  relat- 
ing to  plant  of  this  kind.  In  polyphase  work,  which  is  now  becoming 
of  importance  in  England,  the  author  has  been  especially  careful  to 
make  his  explanations  easy  to  follow. 

Some  change  from  the  original  has  been  inevitable  in  giving  an 
English  dress  to  the  work.  The  illustrations  have  been  revised,  and 
a  number  from  English  firms  have  been  added.  We  here  tender  our 
thanks  to  those  firms  who  have  been  so  good  as  to  provide  us  with 
blocks.  Their  names  are  given  with  the  illustrations. 

We  have  also  to  thank  our  colleague  Mr.  Norman  West,  Demon- 
strator in  Electrical  Engineering,  and  several  of  our  third-year 
students,  for  assistance  in  revising  the  final  proofs. 

W.  W.  HALDANE  GEE. 
CARL  KINZBRUNNER. 

MANCHESTER, 
January,  1903. 

ix 


REVISER'S     PREFACE 


IN  revising  this  book  for  American  readers,  I  have  assumed  that 
not  only  will  interest  in  American  practice  and  apparatus  in  general 
be  equal  to  that  in  foreign,  but  also  that  the  explanations  on  the 
topics  presented  in  the  previous  edition  can  in  this  be  extended  con- 
siderably with  complete  understanding  even  by  persons  but  little 
versed  in  electricity,  satisfying  in  addition  thereby  a  large  class  of 
readers  with  considerable  electrical  experience  or  preliminary  train- 
ing. Certain  subjects  essential  to  American  practice  have  also  been 
added  in  various  parts  of  the  book. 

E.  B.  RAYMOND. 


TABLE  OF  CONTENTS 


CHAPTER  I 

FUNDAMENTAL  PRINCIPLES 

Electrical  Phenomena 1 

Electro-motive  Force 3 

Influence  of  the  Electric  Current  on  a  Magnetic  Needle 10 

Electric  Units — Measurement  of  Currents — Ohm's  Law 14 

The  Calculation  of  Resistance 23 

Other  Forms  of  Ohm's  Law 26 

Internal  Resistance — Drop  of  Potential 27 

Branching  of  Circuits 29 

Cells  in  Series  and  Parallel 31 

Voltmeters 33 

Electrical  Power 34 

Equivalence  of  Electrical  Mechanical  and  Heating  Effects 37 

Electric  Mains 40 

CHAPTER  II 
MAGNETS— MAGNETIC  LINES  OF  FORCE 

Influence  of  a  Magnet  on  an  Electric  Current — Deprez  Instruments 56 

Influence  of  Electric  Currents  on  each  other — The  Electro-dynamometer.  .  60 

Electro-magnets 61 

Induction  65 

Electrical  Machines 68 

CHAPTER  III 
THE  CONTINUOUS-CURRENT  DYNAMO 

The  Ring  Armature 74 

Drum  Armature. 79 

Magnet  System 85 

Self-excitation — Shunt  Dynamo 91 

Series  Dynamo 94 

Compound  Dynamo 97 

Types  of  Dynamos 98 

xi 


xii  TABLE  OF  CONTENTS 

PAGE 

Output  of  a  Dynamo 103 

Multipolar  Dynamos 105 

Armatures  of  Multipolar  Dynamos 109 

Sparking  and  Displaceme.it  of  Brushes 118 

Methods  lor  changing  Direction  of  Rotation 121 

Causes  01  the  Non-excitation  of  Dynamos. 124 

AutOi...*  ic  0111  nu  Regulator 127 

a.  y  of  Dynamos .  129 


CHAPTER   IV 
THE  ELECTRIC  MOTOR 

The  Shunt  Motor 136 

Speed  Regulation 137 

The  Se:  les  Motor 139 

The  Co    ro>md  Motor 143 

Direct    i  of  Rotation  of  a  Motor 145 

Anna          Reaction  with  Motors 149 

Reve,           V  -naratus 150 

Spaik.          i      Starters  and  Shunt  Regulators 153 

Mo  o  ;            er.  ain  Purposes 159 

Electrl      -!.  tion 164 

The  fepi;     .e-General  Electric  Type  M  Control  System 169 

The  Elect ric  Brake 175 

The  Magnetic  Blow-out : 177 

Operating  Troubles  with  Direct-current  Motors 178 


CHAPTER  V 
ACCUMULATORS 

Machines  for  Charging  Accumulators 187 

Battery  Switch 189 

Accumulator  Apparatus 191 

Applications  of  Accumulators 195 


CHAPTER  VI 

WORKING  OF  DIRECT -CURRENT  DYNAMOS  IN  PARALLEL 
Switching  Dynamos  in  Parallel 201 

CHAPTER  VII 
ELECTRIC  LIGHTING 

Glow  Lamps 203 

Arc  Lamps 207 


TABLE  OF  CONTENTS  xiii 

CHAPTER   VIII 
ALTERNATING  CURRENTS 

PAGE 

Properties  of  Angles  concerned  with  Alternating  Currents 217 

Experiments  with  Alternating  Currents , 221 

Current  Strength  and  Voltage  of  an  Alternating  Current 224 

Induction  Effects  of  an  Alternating  Current 225 

1  ransformers 227 

Shape  of  rl  ransformers 229 

Applications  of  Transformers 232 

Ph  se-difference 233 

Vector  Diagrams 239 

Wattmeter — Power-factor 244 

CHAPTER   IX 

ALTERNATORS 

Switching  in  Parallel  of  Alternating-current  Machines — Synchronizer 265 

CHAPTER  X 
ALTERNATING-CURRENT  MOTORS 

Synchronous  Motors 270 

The  Rotary  Converter 276 

Commutator  Motors 281 

CHAPTER  XI 
MULTIPHASE  ALTERNATING  CURRENT 

Induction  Motors — Rotating  Field 283 

Three-pi  ase  Current 290 

Actions  in  Induction  Motors — Squirrel-cage  and  Slip-ring  Armatures 293 

Slip 300 

Single-phase  Induction  Motors 302 

Phase-difference  caused  by  Capacity 305 

The  Reversing  of  Alternating-current  Motors 308 

Faults  with  Alternating-current  Motors 311 

Transmission  of  Multiphase  Currents 314 

Power  in  a  Three-phase  System 322 

Synchronizer  for  Multiphase  Machines 324 

CHAPTER  XII 
HIGH  TENSION 

Lightning  Arresters 330 

Switchboards 334 

INDEX.  .                                                                                                           .  339 


ELECTRICAL   ENGINEERING 


CHAPTER   I 


FUNDAMENTAL    PRINCIPLES 


Electrical  Phenomena 

IP,  in  a  gkss  vessel,  filled  with  water  and  dilute  sulphuric  acid,  in 

the  proportion  of  about  one  part  of  concentrated  acid  to  ten  parts 

of  water,  is  placed  a  plate  of  zinc,  Zn,  (Fig.  1),  and 

a  plate  of  copper,  Gu,  which  are  connected  by  a 

wire,  it  will  soon  be  observed  that  the  wire  gets 

hot.     If  the  contact  at  any  point  of  the  wire 

be  interrupted  so  as  to  make  an  air  gap,  a  spark 

will  be  observed  at  the  moment  when  the  break 

is  made.    This  is  an  electric  spark.    The  name 

"electricity"  is  derived  from   the  Greek  word 

electron,  meaning  amber,  because  this  substance, 

when  rubbed,  produces  electricity.    The  spark 

produced  with  the  apparatus  of  Fig.  1  is  a  very  small  one.    But 

if  we  connect  a  number  of  these  galvanic  cells  in  series,  as  shown  in 


FIG.  1.— Galvanic 
Cell. 


-mmmmmmmmm- 


FIG.  2. — Galvanic  Battery. 

Fig.  2,  taking  care  to  connect  the  Zn  of  one  cell  with  the  Cu  of  the 
other,  then,  on  bringing  together  the  last  Zn  and  the  last  Cu,  sparks 


ELECTRICAL  ENGINEERING 


of  great  length  may  be  obtained,  and  long  spirals  of  motal  —  as 
shown  in  the  illustration  —  may  be  raised  to  red  heat,  and  may 
even  be  melted.  We  call  such  an  arrangement  of  cells  a  galvanic 
battery. 

From  the  heat  produced  in  the  circuit  we  might  infer  that  some 
kind  of  motion  exists  in  the  circuit.  We  know  from  experience  that 
whenever  a  body  is  set  in  motion  by  a  force,  part,  or  in  some  cases 
the  whole,  of  the  force  is  expended  in  overcoming  the  frictional 
resistance  and  producing  heat.  If,  for  example,  some  bricks  slip 
down  an  inclined  plank,  the  latter  becomes  quite  hot,  especially  if 
the  bricks  follow  each  other  very  quickly.  Again,  when  a  train 

E  asses  along  the  rails,  the  temperature  of  the  rails  is  raised,  and  the 
ister  the  train  the  greater  the  amount  of  heat. 

In  the  case  of  the  battery  we  may  imagine  a  motion  in  the 
connecting  wire,  and  the  faster  this  motion  the  hotter  the  wire  be- 
comes.    We   shall    cs.ll  this  motion  which  cannot 
be  seen,  and  is  known  from  its   effects   only,  an 
electric  current. 

Now  let  us  wind  the  wire  connected  with  the 
battery  in  the  form  of  a  spiral,  and  slip  it  over  a 
rod  of  soft  iron  (technically  called  a  core),  as 
shown  in  Fig.  3,  then  the  iron  becomes  strongly 
>  _  magnetic.  But  if  we  break  the  battery  circuit 
anywhere,  so  that  no  current  can  flow,  the  core 


.  —  An  Electro-  will  at  once   lose   its  magnetism.     We   conclude, 
magnet.          then,  that  the  electrical  current  has  the  power  of 
converting    an    iron    bar    into    a    magnet.      The 
arrangement  just  described  is  termed  an  electro-magnet. 

Again,  if  we  take  a  bobbin  wound  with  wire  and  pass  a  current 

through  it,  we  shall  find  that  a  light- 
piece  of  iron  will  be  attracted  into  the 
interior  of  the  bobbin  (see  Fig.  4). 
We  learn  from  this  experiment  that  a 
coil  round  which  a  current  flows  causes 
even  the  space  it  encloses  to  be  a 
magnet. 

Another  important  experiment  is  to 
send  the  electrical  current  through 
acidulated  water.  Immediately  the 
connection  to  the  battery  is  made  the 
water  behaves  as  if  it  were  boiling,  and 
bubbles  of  gas  rise  up  the  wires  and 
escape  into  the  air.  With  the  apparatus  shown  in  Fig.  5  the  gases 
may  be  collected.  The  tubes  are  first  filled  with  water,  and  then 
inverted  over  the  wires,  which  should  be  of  platinum.  One  gas  is 
oxygen,  and  it  will  be  found  that  this  escapes  from  the  end  of  the 


4. — iron  attracted  by 
Electro-magnet. 


FUNDAMENTAL   PRINCIPLES 


wire  connected  with  the  copper  plate;    the  other  gas  is  hydrogen, 

and  it  will  be  double  in  volume 

to  the  oxygen.     Now  collect  both 

gases   in   one   tube,  and   bring   a 

lighted  match  to  the  mouth  of  the 

tube,  when  the  gases  will  instantly 

combine  with  explosion,  and  form 

water,  which  is  composed  of  oxygen 

and  hydrogen. 

From  the  various  experiments, 
we  learn  that  the  effects  of  the 
current  are  threefold,  namely— 

1.  Heating  and  lighting  effects. 

2.  Magnetic  effects. 

3.  Chemical  effects. 

The  student  must  try  to  re- 
member that,  technically,  the  upper 
end  of  the  copper  plate  is  termed 
the  positive  pole ;  and  that  from 
the  wire  connected  to  it  oxygen  FlG-  ^-Decomposition  of  Water, 
escapes,  as  shown  in  Fig.  5.  The 

zinc  end  is  called  the  negative  pole;  from  the  wire  connected  to 
it  hydrogen  escapes. 

The  electric  current  is  supposed  to  travel  from  the  positive  to 
the  negative  pole. 


Electro=motive  Force 

From  the  foregoing  experiments  we  come  to  the  conclusion  that 
a  force  exists  which  drives  a 
current  along  the  wire.  This  may 
be  readily  illustrated  by  the  help 
of  an  analogy.  Let  there  be  two 
tanks  (Fig.  6)  filled  with  water,  but 
to  different  levels,  and  connected 
by  a  pipe.  Then  the  force  due  to 
the  difference  of  heights  of  the 
water  will  produce  a  motion  of  the 
water  in  the  pipe  in  the  direction 
from  the  high  to  the  low  level, 
and  will  continue  as  long  as  there 
is  a  difference  of  level.  In  the  FIG.  6. — Hydraulic  Analogy, 
case  of  electricity  we  must  imagine 
a  similar  difference  of  pressure  to  cause  an  electric  current.  This 


ELECTRICAL   ENGINEERING 


difference  is  termed  a  difference  of  electrical  potential.  When  two 
metals  of  the  same  kind  are  immersed  in  an  acid  solution  no  differ- 
ence of  electrical  potential  is  produced,  and  therefore,  when  the  metals 
are  connected  by  a  wire,  no  current  results.  But  if  the  metals  are 
of  different  kinds  a  potential  difference  is  produced,  and  a  current 
will  pass  through  a  connecting  wire.  If  the  plates  are  of  copper 
and  zinc,  the  potential  of  the  copper  plate  is  higher  than  that  of  the 
zinc,  and  the  current  therefore  flows  from  copper  to  zinc.  The 
force  causing  the  difference  of  potential  is  usually  called  by  electricians 
the  electro-motive  force. 

Again  let  us  refer  to  the  hydraulic  analogy.  The  stream  of 
water,  which  we  compared  with  the  electric  current,  flows  only  as 

long  as  there  is  a  dif- 
ference of  levels.  Now 
let  it  be  considered 
what  would  have  to  be 
done  to  produce  a  con- 
tinuous stream  of  water. 
If  a  pump  be  inserted 
in  the  circuit  (Fig.  7), 
then  the  water  may  be 
forced  to  an  upper 
tank,  and  will  descend 
through  the  opening 
in  the  bottom,  to  be 
again  pumped  up  once 
more.  In  the  electrical 
circuit  we  have  a  similar 
action.  When  two  dif- 
ferent metals  are  im- 
mersed in  acid,  and  one 
FIG.  7.— Production  of  Water  Current.  of  them  is  acted  upon 

by  the  acid,  a  continual 

difference  of  potential  is  produced,  the  chemical  action  here  producing 
the  necessary,  as  it  were,  pumping  action. 

An  electro-motive  force  may  be  produced  by  other  than  chemical 
means,  and  we  shall  show  later  that  electro-motive  force  may  be 
obtained  by  mechanical  power.  The  galvanic  cell  is  only  one 
of  several  means  of  producing  an  electric  current. 


FUNDAMENTAL  PRINCIPLES  5 

In  electrical  engineering  galvanic  cells,  such  as  zinc -copper  cells, 
are  hardly  ever  used  as  current-generators,  but  the  current  is  gen- 
erally produced  by  dynamos.  A  most  important  part  of  a  dynamo 
is  the  magnetic  system,  and  we  have  therefore  to  deal  in  some  detail 
with  the  properties  of  magnets. 

There  are  different  forms  of  magnets,  such  as,  for  instance,  bar 
magnets  (Fig.  8),  horseshoe  magnets  (Fig.  9),  magnetic  needles 


FKJ.  8. 
Bar  Magnet. 


FIG.  9. 
Horseshoe  Magnet. 


FIG.  10. 
Magnetic  Needle. 


(Fig.   10).     With  a  freely  movable  magnetic  needle,  such  as  shown 
in  Fig.  11,  a  characteristic  property  can  be  observed.     If  there  are 


FIG.  11.— Pivoted  Magnetic  Needle. 

no  other  forces  acting  on  this  needle,  it  sets  itself  in  a  certain  direc- 
tion, one  of  its  ends  pointing  towards  the  north.  This  end  is  called 
the  north  pole;  the  other  end,  pointing  towards  the  south,  is  called 
the  south  pole  of  the  needle. 

To  distinguish  the  poles  of  a  magnetic  needle  from  each  other, 
the  north  end  or  half  is  generally  marked  or  coloured. 

We  explain  the  above  phenomenon  by  imagining  that  the  earth 
exerts  upon  the  needle  a  force  which,  like  the  force  of  gravity,  gives  to 
a  suspended  rod  a  certain  definite  direction,  viz.  vertically  down- 
wards. 


6  ELECTRICAL   ENGINEERING 

We  can  deflect  the  needle  from  the  direction  in  which  it  has 
settled  under  the  influence  of  the  earth,  by  placing  near  to  it  another 
magnet.  If  we  place  the  north  poles  of  two  freely  movable  needles 
near  each  other,  then  they  repel  each  other.  The  same  is  the  case 
with  the  two  south  poles.  On  the  other  hand,  a  north  pole  of  one 
and  a  south  pole  of  another  needle  attract.  Hence  the  rule,  "  Like  poles 
repel,  unlike  attract." 

We  can  observe  similar  repelling  and  attracting  effects  by 
approaching  the  magnetic  needle  to  the  poles  of  any  magnet,  and  we 
are  able  to  determine  its  poles  by  the  use  of  the  above  rule. 

As  is  well  known,  a  bar  magnet  attracts  soft  iron.  If  a  soft  iron 
rod  be  attached  to  the  north  pole  of  a  magnet,  the  rod  behaves  like  a 
magnet,  i.e.  it  is  able  to  attract  small  pieces  of  iron  and  hold  them 
fast. 

By  the  aid  of  a  magnetic  needle  we  can  convince  ourselves  that  a 
rod  of  iron  adhering  to  a  magnet  possesses  "polarity,"  the  end  of 
the  rod  turned  toward  the  north  pole  of  the  magnet  being  a  south 
pole,  the  other  end  being  a  north  pole. 

Magnets  exert  actions  at  a  distance.  They  do  not  only  held  fast 
pieces  of  iron  which  have  been  brought  to  them,  but  also  attract 
pieces  of  iron  from  some  distance.  The  force  of  attraction  becomes 
smaller  the  greater  the  distance.  By  the  aid  of  very  exact  experi- 
ments the  following  law  has  been  found  :  If  the  distance  is  doubled, 
then  the  force  exerted  is  not  half,  but  the  fourth  part  of  what  it  was 
previously;  if  the  distance  is  three  times  as  great,  then  the  force  is 
only  the  ninth  part;  with  a  tenfold  distance  the  force  is  only  one- 
hundredth,  and  so  on.  Hence  we  can  say  that  the  force  exerted  by  a 
magnetic  pole  decreases  with  the  square  of  the  distance. 

Let  us  now  take  a  large  horseshoe  magnet,  and  proceed  to  hold 
near  it  a  small  magnetic  needle  in  various  positions.  Fig.  12  shows 

the  poles  of  the  magnet, 

g  seen  from  above,  as  well  as 

7J)1*  son  the  different  positions  of  the 

needle.  If  the  needle  is 
near  the  north  pole,  then  we 


n  2         _  ___  _  _        observe  that  it  comes  to  rest 

^  s<s>n     s<>n     (  S  )n<is     with  the  south  end  pointing 

5          °        V^W         to  the  centre  of  the  north 
***s       pole,  whereas  the  north  end 
FIG.  12.  —  Magnetic  Needle  in  Field  of  Large      of  the  needle  is  repelled  in 
Magnet.  the  opposite  direction  (posi- 

tions 1  and  2  in  Fig.  12). 

In  the  opposite  position,  on  the  right  of  the  south  pole,  the  needle 
comes  to  rest  in  a  similar  way,  so  that  its  north  end  points  right 
to  the  centre  of  the  south  pole  (positions  3  and  4).  At  all  points 
between  the  two  poles  (for  instance,  positions  5  and  6),  the  needle 


I 

FUNDAMENTAL  PRINCIPLES  7 

settles  parallel  to  a  line  joining  the  centre  of  the  poles,  its  south 
end,  s,  turning  towards  the  north  pole,  N,  of  the  magnet,  whilst 
the  north  end,  n,  is  attracted  towards  the  south  pole,  S.  Why 
these  directions  are  taken  up  by  the  needle  the  student  will  easily 
understand. 

But  if  we  now  place  the  needle  at  any  other  point  in  the  space 
surrounding  the  poles,  such  as,  fcr  instance,  at  position  7,  it  will 
again  come  to  rest  in  a  certain  direction. 

This  position,  however,  will  be  such  that  neither  the  south 
pole  of  the  needle  turns  directly  towards  the  north  pole  of  the 
magnet,  nor  the  north  pole  turns  directly  towards  the  south  pole 
of  the  magnet. 

To  explain  why  this  should  be,  we  have  to  consider  the  action  of 
each  of  the  magnet  poles  on  each  pole  of  our  needle. 

The  south  pole,  s,  of  the  needle  is  attracted  by  the  north  pole,  N,  in 
the  directi(  n  of  the  simple  arrowr,  and  is  also  repelled  in  the  direction 
of  the  double-barbed  arrow  by  the  south  pole,  S. 

These  two  forces  are  not  equal  to  each  other,  since  the  centre 
point  of  the  needle  is  only  half  as  far  from  the  north  pole  as  from  the 
south  pole,  the  force  exerted  by  N  being  therefore  four  times  as 
great  as  that  by  S.  In  the  figure  this  is  indicated  by  the  length  of 
the  respective  arrows. 

If  there  are  two  forces  acting  on  a  body,  trying  to  pull  it  in 
different  directions,  then  the  body  can  obviously  follow  neither  of  the 
forces  entirely.  The  direction  which  the  body  then  will  occupy  will 
be  between  the  two  forces,  and  that  not  exactly  in  the  middle,  but 
more  towards  the  greater  force. 

Accordingly,  the  magnetic  needle  will  set  itself  in  the  direction 
of  the  arrow  with  three  barbs,  with  its  south  pole  not  pointing  exactly 
to  the  centre  of  the  N  pole.  Exactly  the  same  considerations  may  be 
applied  to  the  n  pole  of  the  magnetic  needle. 

In  like  manner,  for  each  point  in  the  space  round  the 
magnet  a  certain  line  of  direction  of  the  magnetic  force  can  be 
determined. 

A  very  good  way  to  make  this  clear  is  the  following  one: — 

Let  us  take  a  horseshoe  magnet,  fix  it  vertically,  and  over  it  place 
a  sheet  of  thin  cardboard.  By  means  of  a  muslin  bag  filled  with  steel 
filings,  sprinkle  the  filings  lightly  and  very  uniformly  over  the  card. 
Then  tap  the  card  very  gently.  The  steel  filings  will  arrange  them- 
selves in  beautiful  curves  (see  Fig.  13). 

Near  the  poles  we  observe  rays,  which  are  turned  directly 
towards  the  centre;  between  N  and  S  there  are  formed  straight 
lines  consisting  of  the  filings,  but  at  other  places  on  the  cardboard 
the  lines  are  fewer  and  form  wide  bent  arcs,  which  run  from  pole 
to  pole. 

These  figures  may  be  explained  as  follows : — 


ELECTRICAL  ENGINEERING 


FIG.  13. — Magnetic  Curves. 


Each  iron  filing,   when  it  comes  within  the  influence  of  the 

strong  magnet,  becomes 

a  smfn  maene>  whj^ 

can  turn  round  on  the 
card. 

Hence  each  of  the 
iron  filings  will  take 
up  a  definite  position, 
according  to  its  place 
in  the  space,  in  the 
same  way  that  the 
magnetic  needle  did. 
This  position  will  be 
at  the  points  1  and  2 
(Fig.  12),  directly  to- 
wards the  north  pole; 
at  the  points  3  and  4, 
directly  towards  the 
south  pole;  at  the 

points  5  and  6,  along   the  line  joining   the  poles:    but   at  all    the 
other  points  the  position  will  be  an  inclined  one. 

The  small  pieces  of  magnetized  steel  place  themselves  in  a  row, 
and  so  form  continuous  lines.  These  lines  are  absolutely  straight 
in  the  space  between  the  two  poles,  whereas  beyond  they  are 
curves.  These  curved  lines  hence  consist  of  a  great  number  of  short 
straight  pieces  (the  single  filings),  and  each  of  these  short  pieces 
indicates  the  direction  of  the  resultant  force,  which  the  combined 
influence  of  the  north  and  south  pole  produces  at  this  point.  These 
lines  are  called  lines  of  magnetic  force. 

The  lines  of  magnetic  force  do  not  only  indicate  the  direction 
of  the  force  at  each  point,  but  also  give  us  a  measure  for  the 
magnitude  of  this  force.  We  observe  that  there  are  many  lines  in 
the  immediate  neighbourhood  of  the  poles,  and  in  the  space  between 
the  poles,  whereas  elsewhere  the  lines  are  weaker.  How  is  this  to 
be  explained? 

As  we  know,  the  magnetic  influence  gets  smaller  with  decreasing 
distance.  Hence  the  iron  filings  in  the  immediate  neighbourhood  of 
the  poles  r.re  acted  en  with  great  force,  and  some  of  them  are  pulled 
up  against  the  poles.  The  influence  on  the  iron  filings,  which  are  at 
a  greater  distance  from  the  poles,  is  not  sufficient  to  attract  them, 
perhaps  not  even  enough  to  turn  them,  if  the  friction  on  the  cardboard 
is  greater  than  the  resultant  force. 

The  easily  movable  iron  filings  only  can  follow  this  influence, 
and  the  further  outside  we  go,  the  fewer  filings  will  set  themselves 
in  the  direction  of  the  resultant  force.  Thus  the  lines  of  the  mag- 


FUNDAMENTAL    PRINCIPLES 


netic  force  are  getting  weaker  and  less  distinct  the  further  we  are 
from  the  poles. 

We  therefore  learn  that  the  density  of  the  lines  at  any  point 
gives  us  a  measure  of  the  magnitude  of  the  force  acting  at  this 
point. 

We  can  further  attribute  a  third  meaning  to  the  lines  of  force. 
Let  us  imagine  that  it  were  possible  to  have  a  small  magnet  with 
north  magnetism  only. 
(As  a  matter  of  fact 
that  is  impossible,  for, 
if  we  divide  a  bar 
magnet  into  as  many 
pieces  as  possible,  each 
of  these  pieces  would 
still  have  its  north 
and  south  pole.)  If 
the  north  magnetic 
pole  is  brought  into 
the  sphere  of  activity 
of  the  two  poles,  which 
is  called  the  magnetic 
field,  then  it  would  be 
repelled  by  the  north,  FIG.  14.  —Field  of  Magnetic  Force, 

and    attracted   by    the 

south  pole.  If,  now,  this  pole  is  freely  movable,  it  would  always 
follow  the  course  of  one  of  the  lines  of  magnetic  force  (as  shown  in 
Fig.  14),  and  would  travel  from  the  north  pole  towards  the  south 
pole. 

In  the  position  of  7,  in  Fig.  12,  the  particle  of  iron  will  lie  in 
the  direction  of  the  arrows  with  three  barbs.  If  the  iron  be  now 
shifted  the  direction  it  indicates  alters,  and  is  that  of  the  lines  shown 
in  Fig.  14.  The  line  at  any  particular  place  may  be  supposed  to 
enter  the  south  pole  of  the  little  magnet,  and  to  leave  at  its 
north  pole. 

In  order  to  measure  magnetism  as  we  do  any  other  matter,  it 
becomes  necessary  to  define  a  unit  for  it.  The  unit  of  magnetism 
is  an  amount  of  magnetism  which  if  concentrated  in  a  point  would 
exert  a  unit  force  on  a  similar  amount  of  magnetism  also  concen- 
trated at  a  point  and  located  a  unit  distance  away;  i.e.,  1  centimetre 
(2.54  centimetres  equals  1  inch).  The  unit  of  force  used  to  measure 
these  quantities  is  not  the  pound,  but  a  small  fraction  of  it.  It  is 
called  a  dyne  and  is  equal,  approximately,  to  ^-gVjnJ  of  a  pound. 
Thus,  units  of  magnetism  at  unit  distance  away  exert  a  force  of  1 
dyne  upon  each  other. 

It  is  now  assumed  for  convenience  and  uniformity  that  a  unit 
pole  as  described  has  emanating  from  it  one  line  of  force,  as  described 


10  ELECTRICAL  ENGINEERING 

above,  per  square  centimetre  (6.45  square  centimetres  equal  1  square 
inch)  at  unit  distance  away.  Thus,  a  unit  pole  has  streaming  out  from 
it  as  many  lines  of  force  as  there  are  square  centimetres  in  a  sphere 
of  unit  radius,  or  four  times  TT  (the  Greek  letter  TT  is  generally  used  for 
convenience  to  represent  the  number  3.14159).  Since  at  unit  dis- 
tance away  a  unit  pole  exerts  a  unit  force  on  another  unit  pole,  it 
follows  that  one  line  of  force  per  square  centimetre  means  a  unit 
force  on  a  unit  pole  at  the  point  represented  by  this  line  of  force 
density.  Thus,  in  a  magnetic  field,  whenever  the  density  of  mag- 
netic flow  is  equivalent  to  one  line  per  square  centimetre,  a  unit 
pole  there  would  be  acted  upon  by  a  unit  force  of  1  dyne.  In  the 
air-gap  of  a  dynamo,  for  instance,  the  pull  on  a  unit  pole  equals  per- 
haps 10,000,  which  means  from  the  above  definitions  that  in  this 
magnetic  field  a  unit  pole  would  be  pulled  by  a  force  of  10,000  dynes 
or  A°A°A  pounds.  It  means  also  that  the  lines  of  force  in 
this  field  are  at  a  density  of  10,000  per  square  centimetre  or 
10,000X6.45  per  square  inch.  Faraday  advanced  these  units  and 
hypotheses,  and  they  have  been  used  ever  since. 


Influence  of  the  Electric  Current  on  a 
Magnetic  Needle 

Let  us  make  the  following  experiment :   Over  a  straight  horizontal 
wire  hold  a  magnetic  needle  (see  Fig.  15).     If  there  is  no  iron  in  the 
neighbourhood  capable  of  deflecting  the  needle,  the  latter  will  set 
itself  in  a  direction  lying  north  and  south  with  the  n  pole  pointing 
towards  the  north.     Now,  taking  care  that  the  wire  is  parallel  to 
the  needle,  let  a  current  be  sent  through  the  wire.     We  observe  that 
the  needle  is  now  deflected.      If  we  increase    the  strength  of   the 
current,  the  deflection  will  be  greater,  and  with  a  very  strong  current 
the   needle    will    set    itself   nearly    at     right 
angles  to  the  direction  of  the  wire  as  shown 
in   the  figure.     The  deflection   ceases   imme- 
diately  if   we   stop   the   current,   the  needle 
swings    back,    and    after    a    few    vibrations 
comes  to  rest  exactly  in  the  original  direc- 
tion.    Now  let  the  direction  of  the  current 
be    reversed    by    changing    the    connections 
FIG.  15  —Action  of        of     the  wires  with  the  poles  of  the  battery. 
Current  on  Magnet.         jf    ^   n    end    Qf    ^    needle    were    deflected 

to  the  right  hand  in  the  first  case,  it  will 
now  be  deflected  to  the  left  hand. 

Next  let  the  needle  be  held  underneath  the  wire  instead  of  above 


FUNDAMENTAL  PRINCIPLES 


11 


FIG.  16. — Simple  Galvanometer. 


it,  and  we  shall  find  that  the  deflections  are  now  opposite  to  those 
in  the  first  case.  Ampere,  a  celebrated  French  electrician,  studied 
these  phenomena,  and  found  a  very  simple  rule  by  means  of  which 
the  direction  of  the  deflection  may  always  be  predicted.  "If  we 
imagine  a  man  swimming  in  the  wire  with  the  electric  current  and 
so  as  always  to  face  the  needle,  then  the  north  pole  will  be  deflected 
to  the  left  hand  of  the  swimmer."  This  rule  is  called  Ampere's 
Rule. 

Hence,  if  we  hold  the  needle  above  the  wire,  the  swimmer 
must  swim  on  his  back  and  face  the  needle,  whereas  if  the  needle 

is  under  the  wire  the  swimmer 
must  have  his  face  downwards, 
in  order  to  apply  the  rule  cor- 
rectly. 

As  already  stated,  the  de- 
flection is  greater  the  stronger 
the  current.  It  is  also  in- 
creased by  making  the  distance 
between  wire  and  magnet 
smaller,  and  further  by  having 
a  number  of  windings  round 
the  needle  (Fig.  16).  This 
gives  a  number  of  conductors 
above  and  below  the  needle, 

the  current  flowing  to  the  right  in  the  upper  conductors,  and  to 
the  left  in  the  lower  conductors.  Applying  Ampere's  Rule,  we  find 
that  the  action  of  the  upper  wires  will  deflect  the  n  pole  in  towards 
the  paper;  and  in  the  case  of  the  lower  conductors,  where  the 
swimmer  must  lie  on  his  back,  in  the  same  direction.  All  the 
twelve  conductors  therefore  help  each  other  in  deflecting  the 
needle. 

A  number  of  windings  arranged  in  this  way  is  called  a  galvano- 
meter coil,  and  the  tendency  of  the  current  is  to  bring  the  needle  into 
the  axis  of  the  coil.  When  the  deflecting  force  is  far  stronger  than 
the  directing  action  of  the  earth's  magnetism,  so  that  the  latter  is 
practically  without  effect,  then  the  needle  will  be  driven  at  right 
angles  to  the  coil. 

The  force  of  the  coil  depends  on  the  strength  of  the  current  and 
the  number  of  turns  on  the  coil.  A  coil  having  10  turns  with  a 
current  of  1  amp.  flowing  through  it  has  the  same  effect  as  a 
coil  of  100  turns  and  a  current  of  TL-  amp.,  or  one  with  1000  turns 
and  J±-Q  amp.  Hence,  the  product  of  the  two  quantities  is  of  great 
importance,  and  is  called  the  ampere-turns,  which  in  the  above 
examples  is  equal  to  10. 

We  have  seen,  in  the  previous  section,  that  a  freely  movable 
magnetic  needle  always  points  out  the  direction  of  the  lines  of 


12 


ELECTRICAL  ENGINEERING 


magnetic  force,  and  we  must  now  come  to  the  conclusion 
that  the  electric  current  produces  a  magnetic  field  inside 
the  coil  with  lines  of  force  in  the  direction  of  the  axis  of 
the  coil. 

Returning  to  the  experiment  shown  in  Fig.  15,  let 
us  move  the  magnetic  needle  to  various  positions  round 
the  wire,  and  we  shall  find  that  the  lines  of  force  en- 
circle the  conductor  (see  Fig.  17).  The  action  of  the 
conductor  on  the  needle  being  more  vigorous  near  the 
wire,  we  infer  that  the  lines  of  force  are  here  more  numer- 
ous, and  diminish  as  we  are  more  distant  from  the  wire. 
As  to  the  direction  of  the  lines  it  is  the  same  as  the 
handle  of  a  corkscrew,  if  the  current  be  assumed  to  flow 
from  the  handle  to  the  point  of  the  screw. 

In  order  to  see  the  circular  lines  of  force,  place  a 
piece  of  cardboard  horizontally  and  make  a  hole  in  the 
centre.  Through  this  hole  pass  a  copper  wire  which  is 
held  vertically.  Sprinkle  the  card  uniformly  with  fine 
iron  filings,  and  now  send  a  strong  current  through  the 
wire.  On  tapping  the  card  gently  the  filings  will 
arrange  themselves  in  a  series  of  circles,  as  shown  in  f 

Fig.  18.  I 

If  we  wind  wire  in  the  form  of  a  helix,  or  what  is  • 

generally   called  a  solenoid,  then   the  circular  lines  °f  Lines  of  Force 
force  around  each  part  of  the  wire  will  combine  in  a   round  Cur- 
way  which  will  be  understood  from  Figs.  1!)  and  20.        rent. 
Here  several  windings  are  shown  in  cross-section.     In 
the  upper  part  of  the  windings  the  current  flows  to  us  (marked  by  a 
dot),  whilst  in  the  lower  part  it  flows  from  us  (marked  by  a  cross). 
Using  the  above  rule,  we  find  that  the  lines  of  force  flow  in  the  direc- 
tions marked  by  the  arrows.     For  the  sake  of  simplicity,  only  a  single 
Circle  is  drawn  round  each  wire.     We  notice  that,  when  the  lines  of 
force  are  directed  upwards  and  downwards  in  the  neighboring  circles, 
these  destroy  each  other,  and  only  those  parts  of  the  circles  which 
are  situated  inside  or  outside  the  coil  are  effective.     We  have,  there- 
fore, as  a  resultant  only  straight  lines  inside  and  outside  the  solenoid. 
This  is  seen  in  Fig.  20.    As  a  consequence,  if  a  needle  be  placed  within 
the  coil  it  will  tend  to  set  itself  along  the  axis  of  the  coil. 

From  the  action  of  a  current  on  a  magnet  as  found  by  Ampere, 
and  from  the  fact  that  a  free  north  pole  moves  in  the  direction  of 
a  line  of  force,  it  follows  that  the  lines  of  force  are  created  by  a  flow 
of  electricity  (or  a  current).  It  has  been  found  that  these  lines  of 
force  are  concentric  circles  about  the  wire.  A  current  in  a  wire 
entering  this  page  perpendicular  to  it  creates  lines  of  force  in  the 
plane  of  the  page  circulating  around  in  the  direction  of  the  hands 
of  a  watch,  or  right-handed.  Looking  at  the  end  of  an  electro-mag- 


FUNDAMENTAL  PRINCIPLES 


13 


net,  if  the  current  circulates  in  the  direction  of  the  hands  of  a  watch, 
or  right-handed,  the  pole  is  a  south  pole.  If  counter  clockwise,  or  left- 
handed,  it  is  a  north  pole.  From  the  fact  that  a  free  north  pole  moves 
along  the  lines  of  force,  it  naturally  follows  that  when  a  north  pole 


FIG.  18. — Arrangement  of  Filings  round  Current. 


of  one  magnet  is  approached  to  the  north  pole  of  another  it  is  re- 
pelled, since  the  lines  of  force  come  out  from  a  north  pole.  Since 
the  lines  of  force  go  into  a  south  pole,  the  north  pole  is  attracted 


FIG.  19. — Lines  of  Force  of  Helix. 


.  20.— Resultant  Field  of  Helix. 


to  a  south  pole.  This,  therefore,  expresses  the  law  of  attraction 
and  repulsion  of  magnets,  as  well  as  the  influence  of  currents  on 
magnets.  It  is  only  necessary  to  have  in  mind  the  direction  of  a 
north  pole  when  under  the  influence  of  a  line  of  force. 


14  ELECTRICAL  ENGINEERING 

Another  important  law  in  connection  with  lines  of  force  is  that 
concerning  the  production  of  electro-motive  force.  While  current 
and  electro-motive  force  can  be  produced  by  a  galvanic  battery  as 
described,  the  method  employed  in  dynamos  is  that  of  moving  a 
wire  across  a  magnetic  field.  Faraday  discovered  this  law  of  induced 
currents.  He  found  that  if  a  wire  were  moved  across  a  line  of  force  pro- 
duced by  a  magnet,  that  an  electro-motive  force  was  created  in  this  wire, 
and  that  the  greater  the  density  of  lines  of  force  and  the  faster  the  move- 
ment  the  greater  the  electro-motive  force  created. 

Electricians  have  defined  the  unit  of  electric  pressure  as  that 
resulting  from  cutting  100,000,000  lines  of  force  per  second.  It  is 
called  a  volt.  If  200,000,000  lines  of  force  are  cut  per  second,  two 
volts  are  created.  If  200,000,000  lines  of  force  are  cut,  but  two 
seconds  are  consumed  in  cutting  them,  one  volt  is  produced.  Thus 
the  rate  of  cutting  of  lines  gives  the  resulting  voltage  produced. 
In  a  closed  circuit,  therefore,  the  rate  of  change  of  lines  of  force  in  that 
circuit  gives  the  voltage  produced.  This  is  the  essential  principle  of 
the  modern  dynamo.  The  large  iron  circuit  with  its  spools  creates 
the  lines  of  force,  and  the  armature  with  its  revolving  wires  cuts 
these  lines  of  force  and  produces  electro-motive  force.  Since  motion 
is  relative,  the  same  effect  is  produced  if  the  armature  stands  still 
and  the  poles  or  line  of  force  producers  revolves.  This  latter  arrange- 
ment is  used  for  alternators  where  it  is  desirable,  with  the  higher 
voltages  created,  to  have  the  wires  in  which  the  voltage  is  created 
.stand  still,  and  thus  be  easier  insulated  and  free  from  damage  due 
to  vibration  of  rotation. 


Electric  Units— Measurement  of  Currents — 
Ohm's  Law 

It  is  not  possible  to  measure  an  electric  current  directly,  as  in 
the  case  of  a  stream  of  water  we  can  measure  its  quantity.  With 
the  electric  current  we  must  judge  of  its  strength  by  observing  of 
the  effects  of  the  current.  If,  for  example,  we  connect  a  piece  of 
wire  with  the  poles  of  a  cell,  a'nd  the  wire  does  not  get  hot,  and  if 
we  now  remove  the  wire  and  connect  it  across  the  poles  of  a  battery, 
.and  the  wire  now  is  heated,  it  is  quite  clear  that  the  current  has 
been  stronger  in  the  second  case.  Again,  if  a  coil  is  connected 
firstly  with  a  single  cell  and  it  is  found  that  only  small  pieces  of 
iron  are  attracted,  whereas  when  the  coil  is  connected  with  a  battery 
heavy  pieces  are  attracted,  we  infer  that  the  current  is  of  greater 
strength  in  the  second  case.  A  third  example  may  be  taken  from 
~the  decomposition  of  water:  when  an  electric  current  passes  through 


FUNDAMENTAL  PRINCIPLES  15 

it,  the  greater  the  evolution  of  gas  the  greater  must  be  the  strength 
of  the  current. 

In  one  of  these  ways  the  strength  of  an  electric  current  may 
readily  be  determined,  if  only  we  have  a  unit  strength  of  current 
with  which  to  measure  the  effects.  For  this  purpose  the  chemical 
effect  may  be  best  used,  because  the  quantity  of  the  gas  produced 
c^n  be  measured  by  the  help  of  a  graduated  tube.  Electricians  have 
fixed  upon  that  current  as  a  unit  which  will  liberate  in  one  minute 
10.4  cubic  centimetres  of  mixed  gas.  This  unit  is  called  after  an 
e::.inent  French  scientific  man — an  ampere  (often  abbreviated  to 
amp.). 

If,  then,  we  find  that  a  certain  current  iuves  20.8  cubic  centimetres 
of  gas  per  minute,  we  denote  the  current  strength  as  of  2  amperes; 
if,  on  the  other  hand,  the  amount  of  gas  per  minute  be  104  c.c.  then 
the  current  is  10  amps.;  and  so  on. 

Another  definition  of  unit  current  is  as  follows :  Let  the  current 
be  considered  as  flowing  in  a  wire  of  infinite  length  perpendicular 
to  and  passing  through  this  paper  at  some  point.  The  magnetic 
current  of  the  lines  of  force  from  this  current  circles  in  the  plane 
of  the  paper.  The  density  of  the  lines  of  force  (or  flux)  is  greater 
the  nearer  to  the  wire  they  are  located.  Consider  one  of  the  cir- 
cuits whose  length  is  one  centimetre.  Imagine  a  unit  pole  on  this 
circle.  A  unit  current  can  now  be  defined  as  that  current  in  the 

above  circuit  which  will  produce  on  this  pole  a  force  of  •  ^  dynes, 
or,  what  is  saying  the  same  thing,  the  density  of  the  lines  of  force  at 
this  point  is  ~  lines  per  square  centimetre.  This  definition  of  unit 

current  is  the  same  current  as  defined  by  the  gas-freeing  method. 
It  is  called  an  ampere.  An  ampere  denotes  a  flow.  It  means  a 
unit  of  electricity  per  second.  This  quantity  passing  per  second 
is  called  a  coulomb.  Thus,  one  ampere  is  one  coulomb  per  second. 
A  coulomb  can  be  moving  or  standing  still.  An  ampere  means  motion. 
From  this  definition  of  current  an  excellent  proof  of  what  is  called 
magneto-motive  force  can  be  deduced  as  follows :  It  has  been  stated 
that  ampere  turns  create  magnetism  or  flux.  The  term  magneto- 
motive force  or  "driving  power"  for  magnetism  has  been  given  to 
ampere  turns.  The  ampere  turns  per  inch  of  magnetic  circuit  is 
called  magnetizing  force.  Thus,  a  bar  100  centimetres  long,  with  a 
thousand  ampere  turns  acting  upon  it,  has  a  magneto-motive  force 

1000 
of    1000,    and   a   magnetizing   force   of       „„   =10. 

Thus, 

,,         ...      .  magneto-motive  force 

Magnetizing  force =-, 77; — ? —  r — : T- 

length  of  magnetic  circuit 


16  ELECTRICAL  ENGINEERING 

Return  now  to  the  definition  of  unit  current.  A  un!t  current 
exerts  a  force  of  —  on  a  unit  pole  situated  on  a  circle  away  from 

the  wire  such  a  distance  that  the  length  of  the  circle  is  1  cm.  Thus, 
in  Fig.  21  the  current  of  one  ampere  enters  the  paper  and  at  right 
--„  f,  angles  to  it  at  the  point  A.  A  unit 

pole  at   b  is  acted   upon  by  a  force  of 

— ;  or  the  density  of  lines  of  force  in 
,  /  /     \  \      air,  which  is  expressed  by  the  letter  H 

I  /  /          \  •  4JJ. 

by  electricians,  is  — ,   which   is    saying 

the  same  thing.     Under  the  above  con- 
/c'      ditions,   b  is  located  away  from  A  by 

X  ,''  the  distance  —  instead  of  1  cm.  (since 

"  ^  _  - '  Zn 

-™  ~  ~  V,  the  circumference  of  a  circle  =  2?r  times 

the  radius  when  ^  =  3.14159). 

If  b  were  one  centimetre  away  from  A  at  bf  (Fig.  21)  the  flux  density 
H  would  be  less,  since  the  circumference  of  the  circle  b'-c'-d'  =  In  X 
radius,  or  2n X 1  =  2n,  and  the  circumference  of  this  circle  is  the  length  of 
the  magnetic  circuit;  for  the  lines  of  force  produced  by  the  current 
flowing  into  the  paper  at  A  have  their  path  in  various  circles  in  the 
plane  of  this  paper,  and  the  larger  these  circles  the  less  the  flux, 
as  has  been  shown  on  page  12  by  the  iron-filings  experiment.  As 
a  matter  of  fact,  the  flow  of  flux  in  air  with  magnetic  circuits  follows  the 

,      .,        TT  .  ,.       ,,  ampere  turns 

law:    flux  density =H  is  proportional  to  -, = — ~- : — : r, . 

length  of  magnetic  circuit 

Thus,  referring  again  to  Fig.  21,  if  the  flux  density  (or,  what  is  saying 
the  same  thing,  the  force  of  dynes  in  a  unit  pole)  at  b  with  unit  cur- 
rent at  A  =  j^r,  the  length  of  the  magnetic  circuit  b-c-d  being  1,  it 

would  be  T7v^-27r  at  point  b',  since  the  length  of  the  magnetic  cir- 
cuit at  b'  =  2n.  Hence  the  force  with  one  ampere  one  cm.  away 

2  2 

(at  &/)==TA»  or  H  at  b'  one  cm.  away=-rr.    With  /  amperes  the  force 

27 

would  be  yrr,  and,  if  N  turns  were  interlinked  with  the  flux,  the  force 

2IN 
would  be  -r^p  one  centimetre  away.     At  a  distance  T,  since  the 

length  of  the  circles  representing  the  magnetic  circuits  are  propor- 


FUNDAMENTAL  PRINCIPLES  17 

tionate  to  the  distance  away  from  the  point  A,  the  flux  per  square 
centimetre,  or 


Force  H  =         ........    (1) 


We  have  previously  stated  the  following  definition: 

magneto-motive  force 

Magnetizing  force  =  ;  --  rr  —  ?  —        ~T.  -  =  -  ^r,    ...     (21 
length  of  magnetic  circuit 

IN 

or,  at  the  distance  T,  as  shown,  magnetizing  force  =  ^f) 

IN 

or,  rearranging,        T=  —  —  -  j  --  . 

2rX  magnetizing  force 

Substituting  this  value  of  T  in  equation  (1)  we  get 

_2IN27rX  magneti  zing  force 
"~W  ~IN~          ""• 

or  H  =  rrX  magneti  zing  force, 


or  H  =  1.258  X ampere  turns  per  unit  length  of  magnetic  circuit.  This 
formula  expresses  the  law  of  production  of  flux,  and  is  used  in  the 
calculation  of  all  magnetic  circuits. 

There  is  another  value  expressed  by  the  Greek  letter  p,  called  per- 
meability, which  relates  to  magnetic  circuits.  The  value  H  expressed 
above  refers  to  #ir  magnetic  circuits.  It  is  a  fact,  however,  that  with 
a  given  number  of  ampere  turns  acting  upon  a  circuit  more  flux  will 
be  produced  with  circuit  of  one  material  than  another.  Thus,  iron 
will  produce  perhaps  1500  times  as  much  as  air.  The  ratio  of  the 
flux  produced  in  a  material  to  that  produced  if  the  material  were 
air  is  called  the  permeability  of  the  substance,  and  is  designated 
by  the  Greek  letter  //.  The  JJL  for  air=  1.  While  H  means  flux  density 
per  square  centimetre  (or  force  in  dynes  on  a  unit  pole)  with  air 

T> 

only,  B  means  the  same  thing  with  any  material.     Thus  — = /*.  Thus  B 

means  the  flux  density  per  square  centimetre  with  any  material 
except  air.  Since  iron  is  usually  used  for  magnetic  circuits,  B  usu- 
ally means  flux  density  in  iron  per  square  centimetre. 

An  apparatus  for  measuring  the  strength  of  an  electric  current  by 


18 


ELECTRICAL   ENGINEERING 


means  of  the  chemical  effects  is  called  a  gas  voltameter,  after  Volta, 
who  was  one  of  the  first  to  study  electric  effects. 

Although  the  voltameter  will  measure  the  strength  of  an  electric 
current  it  is  rather  troublesome  in  practice,  and  is  therefore  almost 
exclusively  used  for  laboratory  work  and  for  the  testing  of  other 
instruments,  as  we  shall  presently  see. 

Instruments  depending  on  the  magnetic  effect,  as  described  above 
in  the  magnetic  definition  of  unit  currents,  or  the  heating  effect  are 
very  frequently  used.  In  Fig.  22  is  shown  a  current-measurer  of 
the  electro-magnetic  type.  It  essentially  consists  of  a  coil  of  wire  a, 
into  which  dips  a  thin  and  easily  movable  iron  core,  e.  A  pointer,  /, 
is  connected  with  a  bent  lever  in  such  a  way  that  when  the  core  moves 
downwards  the  counter  revolves,  and  its  position  will  be  indicated 
on  a  scale  which  is  not  shown  in  the  figure.  By  the  adjustment  of 
the  little  weights  at  d  and  d,  which  are  movable  along  the  screws 
threaded  on  the  short  arms,  the  zero  position  of  the  index  may  be 
adjusted. 

If  we  send  a  current  through  the  coil  the  core  will  be  drawn  into 
the  interior,  and  the  pointer  will  therefore  move  to  a  new  position; 


J 


FIG.  22. — Electro-magnetic  Ammeter. 


and  the  stronger  the  current  the  greater  will  be  the  deflection  of  the 
pointer.  To  find  the  value  of  the  current  corresponding  to  the 
deflection  it  is  necessary  to  submit  the  instrument  to  the  process 
called  calibration.  For  this  purpose  it  is  necessary  to  connect  the 
instrument  with  the  voltameter,  in  such  a  way  as  to  cause  the 
current  to  flow  simultaneously  through  both.  If  we  now  regulate 


FUNDAMENTAL  PRINCIPLES  19 

the  current  so  as  to  ensure  the  liberation  of  10.4  c.c.  of  gas  per  second, 


FIG.  23.— Hot-wire  Ammeter. 


FIG.  24  — Electro-magnetic  Ammeter 
(The  Electrical  Company). 


then  we  know  that  1  amp.  is  passing,  and  the  position  of  the  pointer 
can  be  marked  accordingly.  The 
current  can  now  be  increased  so 
that  2  amps,  pass  and  the  new 
position  of  the  pointer  be  marked. 
Let  this  process  be  continued  un- 
til the  highest  possible  deflection 
of  the  pointer  is  reached,  when  the 
calibration  will  be  complete.  We 
have  now  what  is  called  an  am- 
meter. 

In  Fig. 23  is  shown  an  ammeter 
depending  on  the  heating  effect  of 
the  current.  It  is  known  as  of  the 
hot-wire  type.  It  consists  of  a  very 
fine  platinum-silver  wire,  hh,  which 
is  fixed  at  the  points  1  and  2,  and 
is  connected  at  the  middle  point, 
3,  to  another  fine  wire,  d.  This 


FIG.  25. — Electro-magnetic  Ammeter 
(Crompton  &  Co.). 


latter  is  wound  around  a  small 

roller,  r,  and  is  kept  continuously 

strained  by  means  of  a  spring,  /.    When  a  current  is  sent  through  the 

wire  hh  gets  heated  and  expands,  and  so  enables  the  spring  /  to  pull 


20  ELECTRICAL  ENGINEERING 

the  wire  d  forward,  which  action  rotates  the  roller  and  moves  the 
pointer  z.  The  instrument  can  be  calibrated  in  a  similar  way  to  the 
electro-magnetic  instrument.  Complete  instruments  of  the  two  kind* 
are  shown  in  Figs.  24,  25,  and  26. 

In  Fig.  26  it  will  be  seen  that  attached  to  the  pointer  and  moving 
with  it  is  a  disc  of  aluminium.   The  upper  edge  of  this  moAres  between 


FIG.  26.— Hot-wire  Ammeter  (Johnson  and  Phillips). 

the  poles  of  a  horseshoe  magnet.  The  purpose  of  this  arrangement  is 
to  damp  the  motion  so  that  the  pointer  is  speedily  brought  to  rest  and 
the  value  of  the  current  quickly  known. 

We  must  now  examine  the  influence  of  the  E.M.F.  in  producing 
a  current.  For  this  purpose  let  us  go  back  to  the  hydraulic  analogy. 
In  the  tube  connecting  the  two  tanks  the  water-stream  will  be  the 
greater,  the  greater  the  difference  of  the  heights  of  the  columns 
of  the  liquid;  but  it  does  not  depend  on  that  alone.  The  resistance 
of  the  tube  must  have  a  considerable  effect.  If  the  tube  is  very  long 
and  the  inside  rough  and  the  bore  small,  then  but  little  water  can 
pass  through  it.  If,  on  the  other  hand,  the  tube  is  short  with  a  large 
bore  of  polished  material,  then  for  the  same  water-driving  force  the 
stream  of  water  must  be  much  greater. 

Exactly  the  same  is  the  case  with  the  electric  current,  for  the 


FUNDAMENTAL   PRINCIPLES  21 

strength  of  the  electric  current  depends  not  only  on  the  E.M.F.,  but  also 
on  the  resistance  of  the  circuit.  Through  a  short  thick  wire  connecting 
the  poles  of  a  cell  a  stronger  current  will  pass  than  if  a  long  and  thin 
wire  be  used. 

This  dependence  of  the  intensity  of  the  effect  on  the  driving 
force  and  the  resistance  is  to  be  met  with,  not  only  in  the  case 
of  the  water-stream  and  of  the  electric  current,  but  also  with  many 
things  in  daily  life.  For  example,  imagine  there  are  two  coun- 
tries with  a  great  difference  of  density  of  population,  then  many 
people  will  emigrate  from  the  country  with  the  denser  population 
to  the  country  with  the  smaller  one.  The  more  crowded  country 
produces,  in  a  manner,  a  stronger  pressure  towards  the  other 
country,  and  the  pressure  difference  can  be  considered  as  the 
driving  force  of  emigration.  But  not  only  has  this  pressure 
difference  an  influence  on  the  emigration,  but  it  is  of  great  im- 
portance what  opposition  is  offered  to  the  flow  of  the  people 
between  the  two  countries.  If  the  countries  are  near  each  other, 
and  a  good  and  open  road  leads  from  one  to  the  other,  then  the  stream 
of  people  which  flows  from  one  country  to  the  other  may  be  a  very 
great  one;  on  the  other  hand,  if  the  countries  are  separated  by 
means  of  high  mountains,  wide  seas,  etc.,  then  the  resistance  to  the 
flow  will  be  greater,  causing  a  corresponding  diminution  of  the  emi- 
gration. If,  finally,  the  boundary  happens  to  be  an  impassable  one, 
then,  although  a  driving  force  may  exist,  yet  the  countries  will  be 
insulated  from  each  other. 

In  a  like  manner,  there  are  some  materials  which  offer  a  very 
small,  others  which  offer  a  very  great  resistance  to  the  electric 
current.  When  the  resistance  is  extremely  great  the  substance  is 
called  an  insulator.  On  the  other  hand,  materials  which  do  not  much 
impede  the  current  are  termed  conductors  of  electricity.  To  this 
latter  class  belong  all  metals:  first  of  all,  silver  and  copper;  then  gold, 
aluminium,  zinc,  platinum,  iron,  tin,  lead,  German  silver,  the  liquid 
metal  mercury;  next  carbon;  and,  finally,  many  solutions,  such  as 
sulphuric  acid  mixed  with  water,  and  salt  solutions. 

Insulators,  or  non-conductors,  include  the  following:  dry  wood, 
silk,  cotton,  india-rubber,  gutta-percha,  asphalt,  oil,  porcelain,  glass, 
dry  air,  and  so  on. 

It  must  be  clearly  understood  that  the  term  conductor  or  insulator 
is  not  to  be  considered  as  an  absolutely  fixed  one;  this  may  be  well 
understood  by  the  reference  to  the  example  of  two  countries.  In 
ordinary  cases  the  impediment  of  great  mountains  will  be  sufficient 
to  stop  the  passage  of  people  between  the  two  countries,  but  cases 
may  arise  in  which  the  driving  force  may  be  so  great  that  nearly 
all  impediments  can  be  overcome.  In  a  like  manner,  materials  which 
may  insulate  at  lower  voltages  may  become  conductors  at  higher 
pressures.  If  we  cover  a  conductor  with  an  insulating  substance  in 


ELECTRICAL  ENGINEERING 

order  to  prevent  the  escape  of  electricity,  it  follows,  from  what  has 
been  just  said,  that  it  will  be  necessary  to  make  the  covering  thicker 
for  higher  than  for  lower  voltages.  For  a  wire  which  is  connected 
with  a  single  cell,  an  insulation  of  a  winding  of  cotton  or  silk  is 
quite  sufficient,  whereas  a  wire  which  is  connected  with  a  generator 
of  some  1000  volts  must  be  covered  with  several  layers  of  india-rubber 
or  other  insulators. 

The  law  which  expresses  the  relation  between  current  strength 
and  electro-motive  force  tells  us  that  the  current  is  stronger  the 
greater  the  E.M.F.,  and  is  smaller  the  greater  the  resistance  of  the 
circuit.  For  resistance  a  unit  is  also  required.  This  may  be  defi- 
nitely fixed  as  follows  :  If  we  have  a  wire  of  any  size,  and  of  any 
material  through  which  flows  a  current  of  1  amp.,  the  difference 
of  potential  between  the  beginning  and  the  end  of  the  wire  being 
equal  to  1  volt,  then  the  resistance  is  equal  to  the  unit  of  resistance 
which  is  called  the  ohm.  A  convenient  abbreviation  for  the  ohm  is 
the  Greek  letter  a>. 

The  ohm  can  be  made  from  any  metal.  To  give  a  definite  idea 
of  its  value  it  may  be  mentioned  that  about  10  feet  of  a  copper  wire 
of  No.  33  S.W.G.,*  which  has  a  diameter  of  T  J¥  th  of  an  inch,  is  1 
ohm  in  resistance.  Again,  60  metres  of  copper  wire  1  square 
millimetre  cross-section  has  a  resistance  of  about  1  ohm. 

To  make  a  standard  resistance  copper  is  not  used,  for  the  reason 
that  it  oxidizes  in  the  air,  and  that  it  is  difficult  to  obtain  the  metal 
quite  pure.  The  best  material  for  the  purpose  is  mercury.  By 
careful  measurements  it  has  been  found  that  a  column  of  mercury, 
1  sq.  mm.  in  section  and  106.3  cm.  long,  has  a  resistance  of  1  ohm.  . 

Since  1  volt  produces  in  a  circuit  of  1  ohm  a  current  of  1  amp., 
therefore  an  E.M.F.  of  10  volts  will  produce  in  the  same  circuit  a 
current  of  10  amps.;  or,  in  other  words,  the  strength  of  a  current 
varies  directly  as  the  electro-motive  force. 

Let  us  now  keep  constant  the  pressure  as  1  volt,  and  vary  the 
resistance  of  the  circuit;  then  through  a  resistance  of  2a>  will  flow 
a  current  of  J  amp.,  through  a  resistance  of  lOaj  will  flow  a 
current  of  T\  amp.,  and  so  on;  in  other  words,  ^he  strength  of 
a  current  varies  inversely  as  the  resistance.  Let  us  next  con- 
sider what  will  be  the  strength  of  a  current  which  is  produced  by 
an  E.M.F.  of  10  volts,  in  a  circuit  of  2aj.  If  the  resistance 
is  \w  the  resulting  current  is  10  amps.;  but  since  the  resistance  is 
twice  as  great,  the  strength  of  the  current  will  only  be  one-half,  or 
5  amps. 

An  E.M.F.  of  110  volts  will  produce  in  a  circuit  of  220  to  a 
current  of  i^#  =  i  amp.  It  is  obvious  from  these  examples  that 


*  S.W.G.  is  an  abbreviation  for  the  Standard,  Imperial,  or  Board  of  Trade 
Wire  Gauge. 


FUNDAMENTAL   PRINCIPLES  23 

the  number  of  amperes  passing  through  a  circuit  is  obtained  by  dividing 
the  number  of  volts  by  the  number  of  ohms  in  the  circuit,  or  — 

Current  strength  =  Electro-motive  force  ^Resistance. 

Expressed  by  the  initial  letters  of  these  words,  we  may  write— 

C  =  E-R 
or  in  the  form  of  a  fraction  — 


This  last  is  the  mathematical  expression  for  the  law  that  we  have 
expressed  in  words  above.  It  is  called,  after  its  discoverer,  the 
Law  of  Ohm. 

The  Calculation  of  Resistance 

To  compare  the  electric  resistance  of  different  materials  it  is  usual 
to  find  the  resistances  of  the  substances  when  all  are  of  the  same 
length  and  cross-section.  If  the  materials  are  in  the  form  of  wires, 
each  of  some  specified  length  and  cross-section,  say  1  m.  in  length 
and  1  sq.  mm.  in  cross-section,  then  the  number  giving  the  resistance 
in  ohms  is  called  in  each  case  the  specific  resistance. 

From  the  data  previously  given,  it  will  be  easy  to  find  the 
specific  resistance  of  copper  and  mercury.  We  know  that  the 
resistance  of  60  m.  of  copper  wire,  1  sq.  mm.  in  section,  has  a 
resistance  of  lo>,  hence  its  specific  resistance  is  -f^co.  Again,  a 
mercury  column  of  about  1.06  m.  and  of  1  sq.  mm.  cross-section 
has  a  resistance  of  lo>,  so  the  specific  resistance  of  mercury  is 


The  following  numbers  show  approximately  the  specific  resistance 
of  most  of  the  important  metals:  — 

Ohms. 

Silver  ............................  0.016  =-fa  about 

Copper  ...........................  0.0167=^  " 

Gold  .............................  0.02  =^  « 

Aluminium  ........................  0.033  =fa  " 

Brass  .............................  0.070  =TV  " 

Iron  ..............................  0.10  =TV  " 

Lead  .............................  0.22  =| 

German  silver  .....................  0.25  =  ^ 

Nickelin  ..........................  0.35  =  J  " 

Mercury  ..........................  0.94  =|f  " 


24  ELECTRICAL  ENGINEERING 

The  numbers  given  above  are  average  numbers,  and  assume  that 
the  temperature  is  about  60°  F.  In  accurate  work  it  is  necessary  to 
state  the  temperature  for  the  reason  that  the  resistance  varies  with 
the  temperature. 

The  law  of  change  of  resistance  with  temperature  is  expressed 
as  follows:  Let  R0=the  resistance  in  ohms  at  the  temperature  of  0° 
Centigrade  (to  get  Fahrenheit  temperature  from  Centigrade,  mul- 
tiply the  Centigrade  reading  by  -f  and  add  32). 

Let  RT= resistance  at  temperature  T;  then  RT=R0(l  +  aT), 
when  a  is  a  constant  depending  upon  the  material.  For  copper 
a  =  .0042. 

[The  units,  metre  for  length  and  sq.  mm.  for  cross-section,  are  very  convenient 
for  practical  work,  and  are  much  used  abroad.  In  England,  where  the  metric 
system  has  not  yet  been  adopted,  common  units  are  the  inch,  foot,  or  yard  for 
length,  and  the  square  inch  for  sectional  area. 

A  system  in  which  centimetre  measure  is  used  is  recommended  for  universal 
use.  To  change  the  above  numbers  in  accordance  with  this  system  it  is  neces- 
sary to  remember  that  1  metre  is  equal  to  100  centimetres,  and  that  1  sq,  cm.  is 
equal  to  100  sq.  mm. ;  then,  from  what  follows,  the  above  numbers  must  be 
divided  by  10,000 ;  thus  the  specific  resistance  of  silver  in  centimetre  measure  is 
0.0000016. — TRANSLATORS.] 

The  purity  of  the  metal  has  an  important  influence  on  the  spe- 
cific resistance.  With  alloys  the  proportions  of  the  metals  will  give 
great  changes  in  the  resistance.  For  example,  there  are  kinds  of 
copper  which  have  a  specific  resistance  of  -^  and  even  greater;  dif- 
ferent kinds  of  the  alloy  nickelin  have  specific  resistances  ranging 
from  J  up  to  J,  while  German  silver  may  vary  from  \  to  £  in  specific 
resistance. 

The  different  materials  used  for  resistances  in  workshops  and 
laboratories  must  therefore  always  be  electrically  tested,  but  for 
approximate  purposes  the  numbers  given  above  may  be  used. 

With  carbon  the  specific  resistance  differs  greatly  with  the  nature 
of  the  carbon.  There  are  some  kinds  with  a  value  of  some  hundreds 
of  ohms,  whilst  carbon  prepared  under  high  pressure  may  be  only 
12. 

From  the  table  which  we  have  given  we  may  readily  calculate 
the  resistance  of  a  wire  of  any  of  the  materials,  if  we  know  the  length 
and  cross-section  of  the  wire.  As  we  have  learnt  from  the  analogy 
of  water  flowing  through  a  tube,  the  resistance  is  greater  the  longer 
the  tube,  and  is  smaller  the  bigger  the  area  of  cross-section.  A 
copper  wire  10  m.  long  and  1  sq.  mm.  cross-section  has  a  resistance 
of  10X^F=-t&>.  A  copper  wire  1  m.  long  and  2  sq.  mm.  cross- 
section  has  only  half  as  much  resistance  as  the  one  of  1  m.  and 
1  sq.  mm.  area;  thus  ^~-2=T^a>.  We  infer  that  a  wire  of 
10  m.  and  2  sq.  mm.  cross-section  must  have  a  resistance  of 

O=^w. 

These  results  we  may  express  in  words  as  follows: — 


FUNDAMENTAL  PRINCIPLES 


25 


The  resistance  of  a  wire  of  certain  cross-section  and  length  is  to  be 
found  by  multiplying  the  specific  resistance  by  the  number  of  metres 
in  length,  and  dividing  by  the  number  of  square  millimetres  in  cross- 
section;  or  — 

Resistance  =  specific  resistance  X  lengths  area  of  cross-section. 
This  may  be  abbreviated  to  — 


where  R  =  resistance  in  ohms;  /=  length  of  the  wire;  a  =  the  cress- 
section  of  the  wire;  K=the  specific  resistance. 

EXAMPLES. 

1.  What  resistance  has  a  copper  wire  1000  m.  long  and  4  sq.  mm.  cross- 
section? 

Applying  the  formula  we  have  — 


2.  There  are  resistance  frames,  as  shown  in  Fig.  27,  of  frequent  use  in  electro- 
technical  work,  for  the  purpose  of  regulating  the  strength  of  an  electrical  cur- 


FIG.  27. — Resistance  Frame. 


rent.     They  are  generally  made  with  coils  of  metal  having  a  high  specific  resist- 
ance.    Let  us  suppose  that  German  silver  be  chosen  as  the  kind  of  wire.     The 


26  ELECTRICAL  ENGINEERING 

question  is:  How  much  wire,  having  a  cross-section  of  1  sq.  mm.,  must  be  used 
to  give  a  resistance  of  100<»? 

Since  German  silver  has  a  specific  resistance  of  },  then  4  m.  of  a  wire  of 
1  sq.  mm.  will  have  a  resistance  of  Ito,  so  that  for  lOOw  we  require  400  in. 
To  place  such  a  length  of  wire  in  a  comparatively  small  space  necessitates  its 
winding  in  spirals  as  shown  in  the  illustration. 

3.  If  in  the  previous  example  we  had  used  a  wire  of  double  the  cross-section, 
then  it  would  be  necessary  to  take  800  m.     On  the  other  hand,  if  the  cross-section 
had  been  halved  the  length  would  only  be  200  m. 

4.  If  a  copper  wire,  2000  in.  long  and  3  sq.  mm.  cross-section,  be  connected  with 
on  E.M.F.  ot  110  volts,  find  the  current  passing  through  the  wire. 

First  we  must  find  the  resistance  of  the  wire.     This  is — 


The  current  strength  is  now  found  by  dividing  the  voltage  by  this  resistance; 
or — 

E     110 
C=TJ=rrT  =  9-9  amPs° 


Other  Forms  of  Ohm's  Law 

We  are  now  able  to  calculate  the  current,  being  given  the  voltage 
and  the  resistance.  It  is,  of  course,  also  possible  to  calculate  the 
resistance  of  a  circuit  if  the  voltage  and  current  strength  be  given. 
If,  for  instance,  the  voltage  be  10  volts,  and  the  current  flowing  be 
2  amps.,  then  we  may  find  the  resistance  of  the  circuit  as  follows: 
If  the  resistance  of  the  circuit  be  la>,  then  the  number  of  the 
amperes  must  equal  the  number  of  the  volts.  On  the  other  hand,  if, 
as  in  our  example,  the  amperes  are  smaller  than  the  volts,  it  is 
obvious  that  the  resistance  is  greater  than  leu.  A  moment's  thought 
will  show  that  it  must  be  five  times  as  great,  or  5co,  for  the  current 
is  one-fifth  of  the  voltage. 

In  like  manner,  if  the  voltage  be  110  and  the  current  be  J  amp., 
then  the  resistance  of  the  circuit  is  -VV£-  =  220o>. 

The  general  rule  is,  then  — 

The  resistance  of  a  circuit  is  to  be  found  by  dividing  the  E.M.F. 
by  the  current  strength;  or— 

Resistance  =  E.M.F.  -f-  Current  strength; 


There  is  still  one  other  way  of  stating  Ohm's  Law.  Suppose  that 
we  know  the  current  and  the  resistance,  and  require  the  voltage. 
Given,  for  example,  a  resistance  of  220  to  and  a  current  of  J  amp. 
What  is  the  voltage?  Since  to  get  with  a  resistance  of  la)  a 
current  of  J  amp.  a  voltage  of  i  volt  must  be  used,  we  argue  that 


FUNDAMENTAL   PRINCIPLES  27 

to  get  the  same  current  with  a  resistance  of  220oj  we  must  increase 
the  volts  220  times,  or  the  volts  must  be  0.5X220  =  110  volts 
Writing  this  so  as  to  apply  in  a  general  way,  we  say— 

E.M.F.  =  Current  strength  X  Resistance ; 
or  E  =  CXR. 

The  three  formulae: 

C=l ••••<« 

R  =  § (2) 

E-CXR       (3) 

really  mean  one  and  the  same  law  in  different  forms  convenient  for 
practical  calculations. 

Internal  Resistance— Drop  of  Potential 

If  we  connect  any  apparatus,  A  (Fig.  28)  by  means  of  copper 
wires  with  the  battery  B,  our  circuit  consists  of  three  parts,  viz.  the 
battery,  the  main  conductors,  and  the  appara- 
tus. Each  of  these  has  a  certain  resistance, 
and  that  of  the  battery  is  called  the  internal 
resistance.  The  sulphuric  acid  or  other  liquid 
that  may  be  used  in  the  cells  has,  when 
compared  with  metals,  a  very  high  specific 
resistance,  so  that  to  prevent  the  internal 
resistance  becoming  too  great  it  is  necessary 
to  have  the  cross-section  of  the  liquid  suit-  PIG.  28. — Simple  Circuit 
ably  large. 

Suppose  that  the  internal  resistance  of  the  battery  is  la>,  the 
resistance  of  the  main  conductors  2cu,  that  of  the  apparatus  3a), 
and  the  voltage  of  the  battery  12.  Then,  since  1+2  +  3  =  6  is 
the  combined  resistance,  the  current  flowing  in  the  circuit  will  be 
•^•=2  amps.  ^  Hence  a  voltage  of  12  is  required  to  give  2  amps,  in 
the  circuit  with  a  total  resistance  of  QOJ.  If  only  the  apparatus 
of  3w  had  been  in  the  circuit,  then  the  pressure  to  produce  the 
2  amps,  would  have  been  but  3X2  =  6  volts.  Now  the  conducting 
wires  having  a  resistance  of  2aj,  a  pressure  of  2X2=4  volts 
will  be  required  for  them.  Again,  for  the  battery  which  has  a 
resistance  of  \a>,  a  voltage  of  1x2  =  2  volts  will  be  wanted.  The 
total  voltage  of  12  is  thus  consumed  in  the  whole  circuit,  but  only 
6  volts  are  usefully  employed  on  the  Apparatus  A.  The  4  vclts 


28  ELECTRICAL   ENGINEERING 

consumed  by  the  conductors  and  the  2  in  the  battery  represent  a 
loss,  or  drop,  of  potential. 

In  consequence  of  the  drop  of  potential  in  the  battery,  there  is 
available  at  the  poles,  not  the  whole  12  volts  that  the  battery 
produces,  but  a  smaller  number.  If  we  measure,  by  the  help  of  a 
suitable  instrument  (to  be  described  later  on)  the  terminal  voltage 
of  the  battery,  we  shall  find  it  to  be  10  volts  only  when  a  current  of 
2  amps,  flows  through  the  circuit,  giving  under  these  conditions  a 
drop  of  pressure  of  12  —  10  =  2  volts. 

The  drop  of  potential  is  larger  if  the  external  resistance  becomes 
smaller.  Thus,  if  we  replace  A  by  an  apparatus  with  leu  instead  of 
3a>,  since  the  combined  resistance  is  now  l+2  +  l=4&>,  the  current 
will  be  \2  =3  amps.  This  current  will  give  a  potential  drop,  in  the 
battery,  of  3X1=3  volts;  in  the  conductors,  of  3X2  =  6  volts; 
and  for  the  new  apparatus,  3X1  =  3  volts.  The  terminal  voltage 
of  the  battery  is,  in  this  case,  12  —  3  =  9  volts. 

Again,  if  we  suppose  that  the  resistance  of  the  external  circuit, 
consisting  of  the  main  conductors  and  the  apparatus,  be  but  laj,  giv- 
ing a  total  resistance  of  2co  and  a  current  of  ^-  =  6  amps.,  this  will 
give  a  fall  of  volts  in  the  battery  of  6X1  =  6  volts,  leaving  only  6 
as  the  terminal  voltage. 

The  student  will  now  perceive  that,  for  a  certain  current  that  may 
be  needed  for  any  purpose,  it  may  be  necessary  to  reduce  the  poten- 
tial drop  in  the  battery.  How  can  this  be  done?  Obviously  by 
decreasing  the  resistance  of  the  battery.  The  largest  part  of  the 
resistance  of  the  battery  is  usually  due  to  the  liquid.  We  may 
diminish  this  by  making  the  way  through  the  liquid  as  short  and  its 
cross-sectional  area  as  great  as  possible.  The  path  may  be  shortened 
by  placing  the  plates  of  the  elements  very  near  each  other.  The 
cross-sectional  area  of  the  liquid  may  be  enlarged  by  making  the 
plates  that  are  immersed  in  the  liquid  as  large  as  we  may  allow.  It 
may  be  remembered  that  cells  with  large  plates  have  exactly  the 
same  E.M.F.  as  cells  with  the  smallest;  but  the  drop  of  potential  is 
for  the  small  plates  greater  for  the  same  current  than  for  the  largest, 
owing  to  the  internal  resistance.  It  will  therefore  follow  that  the 
cell  with  the  larger  plates  must  have  a  greater  terminal  voltage 
— assuming  the  current  is  the  same.  To  take  the  case  of  the 
battery  with  an  internal  resistance  of  la>  and  an  E.M.F.  of  12 
volts,  at  a  current  of  6  amps,  there  will  be  a  potential  drop  of 
6  volts,  and  therefore  a  terminal  voltage  of  6.  Suppose,  now,  that 
the  plates  of  this  battery  be  made  double  the  size,  causing  the 
internal  resistance  to  be  %(u;  then,  with  a  current  of  2  amps.,  the 
potential  drop  of  the  battery  will  be  only  2Xi=l  volt,  leaving  a 
terminal  pressure  of  11  volts.  On  increasing  the  current  to  6  amps, 
the  potential  drop  will  become  6Xi  =  3  volts,  giving  now  a  terminal 
voltage  of  9. 


FUNDAMENTAL   PRINCIPLES 


29 


Cells  with  very  large  plates  are  seldom  employed,  because  they 
are  difficult  to  manufacture  and  inconvenient  in  use.  We  shall 
presently  see  how  we  may  make  one  large  cell  from  a  number  of 
small  ones. 


Branching  of  Circuits 


NWttttH 


A, 

o 


FIG.  29.— A  Series  Circuit. 


We  have  so  far  dealt  with  a  simple  closed  circuit,  so  that  the 
current  coming  from  the  battery  had  to  flow  through  all  parts  of  the 
circuit.  If  in  a  circuit  be  connected 
several  pieces  of  apparatus  At,  A2,  A3,  as 
shown  in  Fig.  29,  having  respectively  the 
Resistances  2,  3,  and  lo>,  then  the  current 
depends  on  the  sum  of  these  values. 
Suppose  that  the  cross-sectional  area  of 
the  battery  and  of  the  conductors  be  so 
great  that  their  resistance  is  practically 
nothing  when  compared  with  the  resistance 
of  the  rest  of  the  circuit,  as  often  is  the 
case  in  practice,  then  we  can  at  once 

obtain  the  current  by  dividing  6,  which  is  the  sum  of  the  resistances, 
into  the  pressure.  If  the  E.M.F.  be  24  volts,  the  current  will  therefore 
be  -2/  =4  amps.,  which  will  be  the  same  at  all  parts. 

But  we  can  group  the  apparatus  in  another  way.  We  can,  for 
example,  as  shown  in  Fig.  30,  connect  At,  A2,  and  A3,  all  to  the  poles 
of  the  battery.  This  is 
called  the  method  of  con- 
necting in  parallel,  whereas 
the  former  way  was  in 
series. 

The  problem  now  to  be 
considered  is  how  to  find 
the  current  strength  in  each 
apparatus.  First  we  will 
find  the  current  through 
Aj.  Since  this  is  connected 
with  a  battery  of  24  volts, 
and  assuming  that  the  con- 
necting wires  and  internal 
resistance  of  the  cells  are 
practically  nil,  then  the  current  through  At  will  be  obtained  by 
dividing  24,  the  E.M.F.,  by  2,  the  resistance  of  the  apparatus;  or, 
writing  this  after  Ohm's  Law, — 


FIG.  30. — Circuits  in  Parallel. 


30  ELECTRICAL   ENGINEERING 

0,=^-^  =  12  amps.; 


Cl  denotes  the  current,  and  Rx  the  resistance  of 
In  like  manner  we  can  write  — 


=    -  =-234-  =  8  amps.; 


amps. 


The  battery  therefore  has  to  deliver  the  current  to  all  four 
•branches  simultaneously,  which  amounts  to  12  +  8  +  24  =  44 
amperes. 

We  may  now  ask:  If  we  had,  instead  of  the  three  parallel  con- 
nected branches,  a  single  outer  resistance  only,  what  must  be  its 
resistance  that  we  may  get  a  current  equal  to  44  amps.?  This 
problem  may  be  readily  solved  by  means  of  Ohm's  Law.  To  produce 
a  current  of  44  amps,  in  a  circuit  with  24  volts  requires  resistance 
in  the  circuit  of  11  =  0.545^.  This  value  is  defined  as  the  resultant 
resistance  of  the  three  branches.  It  is  smaller  than  any  of  the 
three  branch  resistances.  We  may  say,  generally,  that  we  make  a 
•combined  resistance  smaller  by  connecting  in  parallel,  whereas 
the  combined  value  of  resistances  in  series  is,  of  course,  greater 
than  any  of  them. 

To  work  out  the  value  of  any  resistances  in  parallel  we  may 
proceed  as  follows:  —  Imagine  any  voltage,  preferably  that  of  a  single 
volt,  to  which  the  resistances  are  connected.  Then,  taking  the  three 
branches  of  the  above  example  with  resistances  of  2,  3,  and  1  ohm, 
the  respective  currents  will  be  as  follows  :— 

C1==  1  =  0.500  amps. 
C2  =  J  =  0.333     " 
C8=}  =  1.000     " 

Thus  the  total  current  flowing  through  the  three  branches 
is  — 

1.833  amps., 

and  the  combined  resistance  to  replace  the  three  would  be  — 


which  is  the  same  value  as  that  obtained  in  the  previous  calculation. 
We  may,  then,  state  the  law:  — 


FUNDAMENTAL   PRINCIPLES  31 

To  find  the  resultant  resistance  of  a  circuit  consisting  of  any 
number  of  branches  connected  in  parallel  we  imagine  the  branches 
connected  with  a  pressure  of  one  volt,  then  calculate  the  current  strength 
of  each  of  the  branches,  adding  all  these  branch  currents  together. 
The  resultant  resistance  is  then  found  by  dividing  1  volt  by  the  sum 
of  the  currents. 

The  calculation  becomes  much  simpler  if  the  branch  resistances 
are  equal.  If  we  had,  for  instance,  two  branches,  each  with  a  resist- 
ance of  Wco,  the  current  in  each  branch,  if  connected  with  a 
voltage  of  1,  would  be  y1^  amp.,  and  the  combined  currents  would 
be  -f$=:z  amp.,  giving  by  our  rule  a  resultant  resistance  of  5co,  which 
is  half  that  of  the  branches.  In  the  same  way,  the  combined  resist- 
ance of  10  branches,  each  with  a  resistance  of  10o>,  will  be  laj,  i.e.  the 
tenth  part  of  any  one  of  them,  and  so  on. 


Cells  in  Series  and  Parallel 

The  cells  of  a  battery  may  be  connected  in  series  or  parallel,  and 
the  effect  on  the  internal  resistance  is  exactly  the  same  as  we  have 
found  for  the  external  part  of  the  circuit.  When  two  cells  are  con- 
nected in  series  the  combined  resistance  is  twice  that  of  a  single  cell, 
and  when  they  are  in  parallel  the  resultant  resistance  is  half  that  of 
a  single  cell. 

But  in  addition  to  the  resistance,  the  effect  on  the  E.M.F.  must 
be  thought  of.     In  Fig.  31  are  shown  two  cells  in  series,  the  copper 
of  the  second  cell  being  connected  with  the  zinc 
pole  of  the  first  cell  by  means  of  a   wire,   the 
external   circuit  being   connected    with    the  end 
poles.     The  effect  of  this  arrangement  is  to  add 
the  pressures  of  the  cells  so  that,  if  the  cells  are 
«qual  in  pressure  and  each  of  one  volt,  there  will 
fae  two   volts  available  for  producing  a   current 
through  the  circuit.     In  the  same  way  a  battery    FIG.  31.— Two  Cells 
of  100  cells  would  give  a  voltage  of  100  times  that  in  Series, 

of  a  single  cell. 

We  will  next  consider  the  grouping  of  the  cells  in  parallel. 
Fig.  32  shows  two  cells  so  arranged  having  the  two  copper  plates 
•connected,  and  also  the  two  zinc  plates.  On  joining  a  wire  from  the 
positive  poles  and  the  negative  poles  to  an  outer  circuit,  we  have 
the  effect  of  a  single  cell  of  double  size. 

The  voltage  of  this  battery  is  1,  but  the  internal  resistance  is 
half  that  of  a  single  cell.  If  we  connected  10  cells  in  parallel  in 
this  way,  all  the  zinc  poles  being  joined  together  and  the  same  with 
all  the  copper  poles,  then  the  pressure  will  be  as  before,  1  volt; 


32 


ELECTRICAL  ENGINEERING 


but   the   internal   resistance   will   only   be   the   tenth    part    of    a 
separate  cell. 

If  no  connection  be  made  to  an  external  circuit  no  current  can 
flow  if  the  cells  are  equal  in  voltage.     On  looking  at  Fig.  33,  where 


FIG.  32 — Two  cells  in 
Parallel. 


FIG.  33. — Cells  in 
Opposition. 


the  arrows  show  the  direction  of  the  pressures  in  the  two  cells,  we 
see  that  the  first  cell  tends  to  send  a  current  in  the  direction  of  the 
simple  arrow,  whilst  the  second  cell  would  give  a  current  as  shown  by 
the  feathered  arrow.  These  arrows,  pointing  in  opposite  directions, 

show  that  the  E.M.F.'s  of  the  cells 
are  in  opposition,  and  therefore  no 
current  can  flow,  just  as  a  cart  cannot 
be  moved  by  two  equally  strong 
horses  that  are  pulling  in  opposite 
directions. 

Let  us  now  connect  the  poles  with 
an  outer  circuit;  then,  as  shown  in 
Fig.  34,  one  cell  tends  to  send  a 
current  in  the  direction  of  the  simple 
arrow,  and  the  second  cell  in  the 
direction  of  the  feathered  arrow,  both 
arrows  having  the  same  direction  in 
the  outer  circuit.  Here  the  cells  do 
not  oppose,  but  assist  each  other  in 
driving  a  current  through  the  outside 
circuit.  If  each  cell  supplies  5  amps., 
then  the  current  used  in  the  external 
circuit  will  be  10  amps.  Again,  if  we 

have  10  equal  cells  in  parallel,  the  current  in  any  of  the  cells  will 
be  one-tenth  part  of  the  outer  current. 

When  is  it   desirable   to   arrange   the   cells   in   parallel?     The 


FIG.  34. — Cells  jointly  supplying 
an  Outer  Circuit. 


FUNDAMENTAL   PRINCIPLES 


33 


answer  is  derived  from  the  following  considerations:  If  the  external 
resistance  is  a  great  one,  then  a  large  E.M.F.  is  needed  to  produce 
a  current  of  a  certain  strength.  To  secure  such  a  pressure  it  is 
necessary  to  connect  a  number  of  cells  in  series.  But  in  doing  this 
we  have  at  the  same  time  increased  the  internal  resistance.  If, 
however,  the  external  resistance  is  very  great  compared  with  the 
internal  resistance,  we  shall  gain  more  from  the  increase  of  vol- 
tage than  we  shall  lose  from  the  increase  of  the  resistance  of  the 
battery. 

If,  on  the  other  hand,  the  outer  resistance  is  comparatively  small, 
then  we  must  dimmish  the  internal  resistance  as  much  as  possible  by 
connecting  the  cells  in  parallel. 

The  strength  of  current  given  by  any  cell  is,  of  course,  limited. 
If,  for  example,  a  cell  has  a  pressure  of  1  volt  and  a  resistance  of  ^co, 
then  it  is  impossible  to  get  a  bigger  current  than  10  amp.,  even  if 
we  short-circuit  the  cell  by  connecting  its  poles  with  a  stout  piece  of 
copper  wire.  Usually  such  a  short-circuit  current  will  destroy  a  cell 
in  a  very  brief  time. 

Even  if  we  connect  100  such  cells  in  series,  we  cannot  get  a 
greater  current  than  10  amps.;  for  although  the  E.M.F.  is  100  volts, 
yet  the  internal  resistance  is  lOOXiV  —  lO^  giving  on  short-circuit 
a  current  of  10  amps. 

On  the  other  hand,  if  these  cells  are  arranged  in  parallel  and 
short-circuited,  then  the  current  obtainable  will  be  100X10  =  1000 
amps.  This  result  may  be  ob- 
tained at  once  by  remembering 
that  the  resistance  of  the  100 
branch  circuits  is  i^-$X^  = 
T  oVs  >  and  with  the  1  volt  avail- 
able the  current  will  be  -5-. /or  = 
1000  amps. 


Voltmeters 

The  voltage  can  be  measured 
with  a  similar  apparatus  to  the 
ammeter.  Fig.  35  shows  one  of 
the  electro-magnetic  type.  It 
is,  of  course,  very  important 
that  the  coil  of  such  an  instru- 
ment take  as  little  current  as 
possible.  It  accordingly  consists 

of  many  windings  of  a  very  fine  wire,  so  that  its  resistance  is  a  very 
high  one.  Although  the  current  is  very  small,  yet  owing  to  its 
passing  so  often  round  the  coil  the  magnetic  effect  may  be  as  great 


FIG.  35. — Voltmeter,  Electro-magnetic 
Type.     (The  Electrical  Company.} 


34 


ELECTRICAL   ENGINEERING 


as  in  the  case  of  the  ammeter,  which  is  supplied  with  a  strong  current 
that  passes  round  only  a  few  times. 

A  hot-wire  instrument  may  also  be  used  as  a  voltmeter.  But 
here,  owing  to  the  very  small  resistance  of  the  short  and  fine  wire, 
that  expands  when  heated,  it  is  necessary  to  put  in  series  with  it  a 
resistance.  This  resistance  consists  of  very  many  windings  of  fine 
wire.  One  or  more  of  these  resistance  coils  are  placed  either  within 


V 


FIG.  36.  —Hot-wire  voltmeter  (Johnson  and  Phillips). 

the  instrument  or  in   separate  boxes.     Fig.   36   shows  a   hot-wire 
voltmeter. 

The  higher  the  voltage  to  be  measured,  the  greater  must  the 
resistance  in  the  circuit  of  the  voltmeter  be  made. 


Electrical  Power 


Let  the  following  experiment  be  made :  Take  a  spiral  of  German- 
silver  wire  of  a  certain  length  and  a  certain  cross-section,  and  connect 


FUNDAMENTAL   PRINCIPLES 


35. 


it  with  a  source  of,  say,  10  volts.  Then,  if  the  voltage  is  sufficient,  the 
spiral  will  get  hotter  and  hotter;  and  if  the  size  of  the  wire  be  rightly 
selected,  a  certain  temperature  will  be  reached  which  will  remain 
constant.  We  must,  therefore,  conclude  that  now  the  wire  radiates 
as  much  heat  to  the  surrounding  air  as  is  produced  by  the  electric 
current. 

The  most  common  application  of  this  principle  is  the  electric 
incandescent,  or  glow,  lamp. 

In  a  closed  glass  bulb  exhausted  of  air  a  carbon  filament  is  fixed, 
and  its  ends  are  connected  to  two  metallic  contacts.  Such  a  lamp  is, 
shown  in  Fig.  37.  On  arranging  these  con- 
tacts so  that  they  receive  an  electrical  pressure 
of  a  certain  height,  an  electric  current  will 
flow  through  the  carbon  and  raise  it  to  white 
heat. 

The  most  usual  type  of  lamp  has  a  candle- 
power  of  sixteen,  and  is  made  for  a  supply  at 
110  volts.  It  has  a  resistance  of  220 to,  and 
hence  a  curent  of  JJ-§-  =  0.5  amp.  will  flow 
through  it',  which  is  just  sufficient  to  keep  the 
filament  at  white  heat. 

If  we  wish  two  glow  lamps  to  give  light  we 
may  connect  them  in  series  and  provide  a 
voltage  of  220,  when  through  each  will  pass  a 
current  of  0.5  amp.;  the  resistance  of  the  two 
filaments  being  440&>. 

The  same  result  can  be  produced  in  another    „ 
way.     Let  the  two  lamps  be  placed  in  parallel 
with  110  volts.     The  resulting  resistance  will 
now  be  110&>,  so  that  the  total  current  will  be 
through  each  carbon  will  flow  the  necessary  J  amp. 

On  comparison  of  these  two  cases  we  see  that  the  same  heating- 
effect  is  produced  with  220  volts  and  0.5  amp.  as  with  110  volts 
and  1  amp.  This  shows  that  the  heating  effect  is  dependent,  not 
only  on  the  voltage  or  current,  but  on  the  product  of  these  two 
quantities.  This  product,  vo Its  X  amperes,  is  defined  as  the  electrical 
power.  The  unit  volt-ampere,  being  rather  long,  is  abbreviated  to 
Watt,  after  the  name  of  the  famous  improver  of  the  steam-engine. 

The  power  of  the  current  in  the  case  of  the  lamps  which  we  have, 
been  just  considering  is — 

110  voltsX0.5  amp.  =  55  volt-amperes  =  55  watts. 

Generally  we  can  write  the  equation — 

Power  =  Electro-motive  force  X  Current  strength ; 


or  Glow  Lamp. 
1  amp.,  and 


36  ELECTRICAL   ENGINEERING 

or,  in  symbols,  — 


If  it  so  happens  that  the  voltage  and  the  resistance  only  be  known, 
then  it  is  easy  to  find  the  power  by  the  aid  of  the  preceding  rule,  first 
finding  the  current  from  — 

r-E 
R 

hence  — 


Or,  if  the  voltage  and  resistance  be  given  to  find  the  power,  we  must 
multiply  the  number  of  volts  by  itself,  and  divide  the  product  by  the 
number  of  ohms. 

To  take  the  case  of  the  glow  lamp  above,  to  find  the  power  from 
the  voltage  of  110  and  the  resistance  of  220  we  should  have  — 

P  =  1  10  X  1  10  +  220  =  55  watts, 

the  same  result  as  before. 

Again,  if  the  current  and  resistance  be  given,  we  can  calculate  the 
power  from  the  third  form  of  Ohm's  Law  — 


By  substituting  this  value  of  the  E.M.F.  in  — 

P  =  EXC 
we  get  — 

P=CXRXC 
or  — 

P=CxCXft 

that  is,  we  find  the  number  of  watts  by  multiplying  the  number  of 
amperes  by  itself,  and  then  by  the  ohms. 

Thus,  for  the  case  of  the  glow  lamp,  we  have  — 

0.5X0.5X220  =  55  watts, 

—  this  being  the  same  result  as  we  previously  obtained,  proving  tht>_ 
the  three  methods  are  equivalent. 

For  the  value  CxC  the  abbreviation  C2  (C  squared  or  C  raised 
to  the  second  power)  is  generally  used.  The  exact  meaning  of  this 
will  be  understood  from  the  following  considerations:  — 

If  we  have  to  determine  the  area  of  a  square  we  can  do  so  by 
dividing  its  sides  into  equal  parts,  each  of  which  is,  say,  1  inch  long; 
then,  by  drawing  lines  through  these  marks  horizontally  and 


FUNDAMENTAL   PRINCIPLES 


37 


vertically  the  whole  area  of  the  square  will  be  split  up  into  a  number 
of  smaller  squares,  each  having  an  area  of  1  sq.  inch.  By  counting 
the  number  of  these  we  find 
that  a  square  whose  sides  are 
2  inches  long  has  an  area  of 
2X2=- 4  sq.  inches,  whilst 
one  of  3-inch  side  has  an  area 
3X3  =  9  (see  Fig.  38).  To 
multiply  a  number  by  itself 
is  thus  the  same  as  to  deter- 
mine the  area  of  a  square 
whose  side  length  is  equal 
to  this  number.  The  three 
formulae  given  above  may 
therefore  be  written — 


FIG.  38. 


P  =  EC 

-I 

P=C2R 


(1) 
(2) 
(3) 


The  fact  that  the  power  depends  both  on  the  voltage  and  on  the 
current  strength  may  be  understood  by  reference  to  the  hydraulic 
analogy.  The  effect  of  flowing  water  depends,  not  only  on  the 
pressure  or  water-driving  force  produced  by  a  difference  of  levels, 
but  also  on  the  quantity  of  water  per  second,  which  corresponds  to 
the  strength  of  the  electric  current.  A  pound  of  water  flowing  down 
a  height  of  10  feet  will  be  able  to  do  ten  times  the  amount  of  work 
— such  as  driving  a  water-wheel — as  a  pound  of  water  flowing  down 
a  height  of  one  foot  only.  The  effect  of  flowing  water  is  thus  pro- 
portional to  the  product  of  the  number  of  pounds  of  water  and 
number  of  feet.  This  product  is  called  foot-pounds.  The  power  of 
a  waterfall  is  estimated  by  the  number  of  foot-pounds  per  minute  or 
second. 


Equivalence  of  Electrical  Mechanical  and 
Heating  Effects 

Heat  can  be  produced  both  electrically  and  mechanically.  A 
vessel  containing  water  may  be  heated  by  placing  in  it  an  insulated 
spiral  of  wire  which  is  connected  with  a  current  generator.  Heating 
of  water  may  also  be  caused  by  dropping  from  a  height  a  stone  into 
it;  or  a  better  example  for  our  purpose  would  be  the  case  of  sand 
flowing  continuously  into  water.  In  this  case  there  will  be  a  gradual 


38  ELECTRICAL   ENGINEERING 

heating  of  the  water,  just  as  in  the  case  of  an  electric  current.  After 
some  time  the  temperature  of  the  water  will  cease  to  rise,  showing 
that  the  heat  produced  by  the  flowing  $and  is  now  equal  to  the  heat 
radiated  to  the  surrounding  bodies,  sucn  as  the  vessel,  air,  etc. 

Seeing  that  both  by  mechanical  and  electrical  means  heat  may  be 
produced,  the  question  may  be  asked1:  How  many  mechanical  units 
correspond  to  the  electric  unit,  or  ^  watt?  This  question  may  be 
answered  by  the  help  of,  the  following  experiment.  Take  two  equal 
vessels  containing  equal  quantities  of  water,  and  let  one  be  heated 
electrically,  and  the  other  mechanically  by  the  use  of  falling  sand, 
so  that  the  amount  of  heat  passing  in  is  the  same  in  the  two  cases  as 
shown  by  a  thermometer.  Let  pe  voltage  and  current,  and  the 
rate  at  which  the  sand  falls  from  a  known  height,  be  noted.  By 
means  of  these  values,  and  experiments  of  a  similar  kind,  it  has 
been  found  that  — 

One  foot-pound  per  second  =  1.356  watts. 

We  shall,  in  subsequent  pages,  deal  with  machines  which  enable 
us  to  convert  mechanical  into  electrical  power.  We  may,  for 
example,  have  an  electrical  machine  driven  by  a  water-wheel  or  a 
turbine;  then,  if  we  know  the  height  of  the  fall  and  the  amount  of 
water  per  second,  the  electrical  power  can  be  estimated.  If,  for 
instance,  1000  Ibs.  of  water  falls  down  a  height  of  5  feet  each  second, 
then  the  mechanical  value-  of  this  will  be  5000  ft.-lbs.  per  second. 
If  it  were  possible  to  convert  all  the  mechanical  into  electrical  power, 
or,  in  other  words,  if  there  were  no  losses,  then  the  number  of  watts 
produced  would  be  — 

=  3687  watts. 


As  a  matter  of  fact  the  number  of  watts  would  be  considerably  smaller 
than  this. 

Being  given  the  number  of  3687  watts,  we  can  calculate  the 
number  of  16-candle-power  lamps  that  could  be  lighted.  If 
each  lamp  required  55  watts,  then  the  number  would  be  -||-  =  67 
nearly. 

It  is  not  the  general  custom  with  engineers  to  state  the  output  of 
a  machine,  such  as  a  turbine,  or  steam  or  gas  engine,  in  foot-pounds 
per  second,  because  of  the  large  numbers  that  would  have  to  be 
used.  Instead,  a  much  greater  unit  called  the  horse-power  (abbre- 
viated H.P.)  is  used.  A  horse-power  is  defined  as  that  rate  of  doing 
work  that  is  equal  to  raising  33,000  Ibs.  1  foot  high  in  1  minute. 
This  is  equal  to  an  effort  of  s^op  =  550  ft.-lbs.  a  second.  This 
unit  was  introduced  by  James  Watt,  and  has  since  been  used  by 
British  engineers  in  stating  the  power  of  engines.  It  is  supposed  to 


FUNDAMENTAL   PRINCIPLES  39 

represent  the  power  of  a  very  strong  horse  when  working  very  hard. 
The  equivalent  electrical  power  is — 

550  X 1 .356  =  746  watts  nearly. 

If  we  know  what  is  the  output  of  any  machine  given  in  H.P., 
then  we  can,  if  it  is  coupled  to  an  electrical  generator,  calculate  the 
electrical  output,  providing  that  no  losses  be  taken  into  account. 
Thus  a  steam-engine  of  1  H.P.  would  drive  a  dynamo  giving  an 
output  of  746  watts.  The  real  electric  output  due  to  losses  of  friction, 
etc.,  is,  of  course,  less  than  this,  and  generally  varies  between  700  to 
600  watts  per  H.P.,  according  to  the  size  of  the  machines.  For  small 
machines  it  comes  down  to  500  watts.  These  numbers  correspond 
with  from  9  to  13  lamps  per  H.P.,  if  55- watt  lamps  be  used. 

A  small  dynamo,  for  example,  driven  by  a  5-H.P.  steam-engine, 
could  feed  about  5X10  =  50  lamps;  whilst  a  large  one  of  about 
200  H.P.  would  supply  200X13  =  2600  lamps  each  of  16-candle- 
power.  On  the  other  hand,  if  this  200-H.P.  engine  had  been  used 
for  driving  a  pump  that  was  perfectly  efficient,  then  it  would  lift 
200X550  =  110,000  Ibs.  of  water  per  second,  a  height  of  1  foot. 

The  relation  that  exists  between  the  electric  and  heating  effects 
must  now  be  studied.  First,  the  unit  of  the  quantity  of  heat  must 
be  defined.  This  unit  is  called  the  calorie;  it  is  equal  to  the 
quantity  of  heat  which  is  necessary  to  raise  1  grm.  of  water  from 
zero  to  1°  on  the  Centigrade  scale.  Hence,  to  raise  1  kg.  of  water 
from  zero  to  boiling-point  (100°  C.)  will  require  1000X100=100,000 
calories. 

The  relation  between  the  two  effects  can  be  determined  by  the 
following  experiment:  A  vessel  containing  a  known  quantity  of 
water  is  heated  by  means  of  an  insulated  wire  through  which  a 
current  is  flowing,  and  the  temperature  ascertained  by  means  of 
a  thermometer  before  the  current  has  been  passed,  and  also  after  it 
has  been  flowing  for  a  known  number  of  seconds.  At  the  same  time 
the  voltage  and  the  current  are  noted.  The  quantity  of  heat,  and  the 
watts,  and  time  are  thence  known.  The  result  is,  that  1  calorie 
corresponds  to  4.2  watts  per  second;  or  the  heat  equivalent  of  1  watt- 
second  [1  watt-second  is  called  a  "  joule"]  is — 

2^  =0.24  calories  approximately. 

We  can  now  readily  calculate  the  quantity  of  heat  produced  by 
a  16-candle-power  glow  lamp  in  one  hour.  Since  in  one  second 
55  watt-seconds  are  used,  then  in  one  hour  the  number  will  be 
55X60X60  =  198,000,  so  that  the  number  of  calories  will  be 
198,000X0.24  =  47,520. 

On    flowing    through    the    carbon    filament   the    current   causes 


40  ELECTRICAL   ENGINEERING 

a  temperature  which  rises  until  a  bright  white  heat  is  reached. 
After  this  the  temperature  remains  constant,  because  the  heat  is 
radiated  as  fast  as  it  is  produced  by  the  current;  or;  in  other  words, 
a  stationary  state  is  reached. 

Let  us  next  ascertain  what  will  happen  if  we  connect  this  lamp 
with  a  voltage  of  150  instead  of  110.  If  we  may  make  the  supposi- 
tion that  the  resistance  of  the  lamp  remains  the  same  as  before,  the 
current  now  will  be— 

Jf  £=0.68  amps., 

and  its  watts  now  become 

150X0.68  =  102  watte. 

The  watts  being  nearly  twice  as  great  as  previously,  a  double  quantity 
of  heat  will  be  produced  per  second,  and  the  carbon  will  reach  a  far 
higher  temperature  than  before,  and  will  give  out  more  light.  This, 
however,  will  not  last  for  a  long  time,  because  the  effect  of  the  high 
temperature  soon  causes  the  filament  to  be  broken.  The  "life"  of 
such  a  lamp  will  thus  be  far  shorter  than  when  the  voltage  is 
normal. 

Again,  if  we  connect  a  110-  volt  lamp  with  a  voltage  of  220  volts, 
then  the  power  required  is  four  times  as  great  as  before,  for  — 


In  this  last  case  the  heat  produced  is  such  as  to  ruin  the  filament 
immediately  on  switching  the  lamp  into  the  circuit. 

As  a  matter  of  fact  the  power  taken  by  the  filament  in  the  two 
cases  of  150  and  220  volts  is  far  higher  than  our  calculations  indicate, 
because  the  resistance  of  carbon  decreases  —  contrary  to  the  case  with 
metals  —  with  a  rise  of  temperature. 


Electric  Mains 

To  lead  the  electric  current  from  the  place  of  generation  to  the 
place  where  it  is  used  we  require  leads  or  mains.  Generally  we 
want  two  mains,  one  leading  in  and  one  out,  just  as  we  must  have 
two  channels,  to  lead  water  to  a  water-wheel  and  to  lead  it  away 
{see  Fig.  39). 

The  first  channel  connects  our  water-wheel  with  the  higher,  the 
other  channel  connects  it  with  the  lower  level.  In  a  like  manner 
the  mains  serve  to  connect  the  electric  apparatus  with  the  positive 
and  negative  poles  of  the  current-generator. 


FUNDAMENTAL   PRINCIPLES 


41 


The  water-channels,  which  are  generally  made  from  earthenware 
or  wood,  always  cause  a  loss  of  motive  force.  If  the  channel  is 
not  tight  enough,  some  of  the  water  will  leak  through  the  cracks. 
This  part  of  the  water  will  not  reach  the  water-wheel  at  all,  thus 
involving  a  direct  loss  of  water.  Further,  a  part  of  the  whole  pressure 
gets  wasted  in  flowing  through  the  channels;  this  loss  is  represented 
in  Fig.  39  by  the  line  h{  for  the  upper  channel,  and  by  the  line  h2 
for  the  lower  channel.  Thus,  the  total  height  difference  of  water 
available  for  driving  the  water-wheel  is  not  H,  but  H  diminished  by 
Thus  the  heights  h^  and  h2  represent  pressure  losses. 


FIG.  39. 


In  like  manner,  we  have  with  electric  mains  losses  of  current 
and  losses  of  voltage  or  potential  drop.  The  former  occur  with 
badly  installed  mains  only.  If,  for  instance,  mains  leading  from 
a  central  electric  station  to  a  group  of  lamps  are  in  several  places 
in  connection  with  the  earth  or  with  damp  walls,  then  the  current 
will  not  only  flow  through  the  lamps,  but  a  part  of  it  will  also 
flow  from  the  positive  wire  to  the  earth,  and  from  the  latter  to 
the  negative  wire,  without  going  through  the  lamps.  The  current 
lost  in  such  a  way  will  be  greater  the  better  is  the  connection 
of  each  of  the  mains  with  the  earth,  and  the  nearer  the  bad  points 
of  the  positive  main  are  to  the  bad  points  of  the  negative  main. 
If  these  be  very  near  each  other,  then  the  resistance  of  the  earth 


42 


ELECTRICAL   ENGINEERING 


between  them  will  be  very  small,  and  a  comparatively  large  current 
will  leak  away. 

To  avoid  such  losses,  and  the  risks  connected  therewith,  mains 
have  to  be  very  well  insulated.     Mains  are  provided  with  a  continuous 

insulating  covering,  or  they  may  be 
left  bare,  but  in  this  case  they  have 
to  be  fixed  on  bell-shaped  insulators, 
as  shown  in  Fig.  40.  They  are 
made  of  porcelain  or  glass  in  the 
shape  of  a  bell,  and  fastened  by 
means  of  insulating  cement  to  an 
iron  bracket.  The  latter  is  fixed  on 
a  mast  or  to  a  wall.  The  conductor 
is  secured  by  wire  to  the  groove  of 
the  insulator.  Porcelain  and  glass 
are  excellent  insulating  materials, 
and  the  wires  fixed  to  the  insulators 
are  therefore  entirely  insulated  from 

the  iron  bracket.  Owing  to  the  special  shape  of  the  insulator  even 
raindrops  cannot  make  an  electric  connection,  because  the  bell-shaped 
part  is  usually  fixed  in  a  vertical  position.  For  extra  high  pressures 
— such  as,  for  instance,  5000  or  10,000  volts,  an  insulator  of  the  shape 
described  would  not  be  srfe  enough.  In  such  cases  double  or  triple 
bells  are  employed,  as  shown  in  Fig.  41 . 


FIG.  40. — Porcelain  Insulator 
(General  Electric  Co.). 


FIG.  41. — High-tension  Insulator. 

This  method  of  running  mains  is  often  used  for  overhead  or  aerial 
lines.     In  fixing  such  lines,  care  must  be  taken  to  avoid   contact 


FUNDAMEXTAL    PRINCIPLES 


43 


of  the  wires  with  each  other,  and  with  other  bodies.  They  must 
not  be  placed  near  trees,  because  the  branches  and  leaves  might  then 
touch  the  wires,  thus  forming  in  damp  weather  a  good  connection 
with  the  earth. 

The  mains  installed  in  the  streets  of  large  towns,  or  within  houses, 
consist  of  insulated  wires  only.  The  method  of  insulation  of  these 
mains  depends,  on  one  hand,  on  the  voltage  of  the  current  which  they 
conduct,  and,  on  the  other  hand,  on  the  position  in  which  they  are 
fixed.  Mains  for  low  voltages,  installed  in  dry  rooms,  may  be  covered 
with  a  thin  layer  of  insulation  only.  In  such  a  case  it  would,  for 
instance,  be  sufficient  to  cover  the  wires  with  a  thin  winding  of  cotton 
or  hemp,  and  to  impregnate  this  winding  with  tar  or  asphalt.  For 
high  voltages  and  damp  rooms,  the  wires  must  be  covered  with  india- 
rubber,  and  several  layers  of  cotton  or  hemp. 

Cables  laid  in  the  earth  or  channels  are  exposed  both  to  the 
influence  of  moisture  and  acids,  and  are  liable  to  mechanical  injuries. 
They  have,  therefore,  to  be  protected  in  addition  to  the  different 
insulation  layers  with  a  lead  covering,  which  is  further  covered  with 
an  insulating  layer.  Protection  against  mechanical  injuries  is 
frequently  guarded  against  by  an  iron  or  steel  armouring,  which 
latter  may  be  protected  against  corrosion  by  a  cotton  or  hemp  network 
impregnated  with  bitumen. 

Fig.  42  shows  a  cross-section  through  a  cable,  containing  both  the 
positive  and  the  negative  wire.  The  circles  in  the  centre  represent 
one  main  surrounded  by  an 
insulating  layer.  The  second 
main  consists  of  a  number 
of  thin  copper  wires  arranged 
in  a  circle.  Next  to  these 
wires  is  an  insulation-layer, 
then  a  lead  covering,  and, 
finally,  an  outer  casing.  As 
the  inner  and  outer  wires 
form  circles  with  the  same 
centre,  the  cable  is  called  a 
concentric  one.  Cables  are 
also  manufactured  in  which 
the  single  insulated  wires  are 
stranded  with  each  other. 

Exact  specifications  relat- 
ing to  the  insulation  and 
laying  of  cables  may  be 
found  in  the  Board  of  Trade 
Regulations,  the  Rules  of  the 

Institution  of  Electrical  Engineers,  and  those  of  Fire  Insurance 
Companies. 

The  losses  of  current  can  be  avoided  by  proper  installation  of  the 
mains.  Losses  of  voltage  cannot  be  avoided,  because  the  mains  have 


FIG.  42. — Section  of  Concentric  Cable 
(Siemens  and  Halske). 


44  ELECTRICAL  ENGINEERING 

in  any  case  a  resistance,  and  thus  a  voltage  drop  must  occur  in  them, 
which  may  be  determined  by  Ohm's  Law. 

Let  us  now  work  out  the  following  example  :  The  distance  between 
a  current  generator  and  a  room  which  is  lighted  by  20  lamps,  each 
of  16-candle-power,  and  connected  with  110  volts,  is  80  yards, 
and  the  cross-sectional  area  of  the  wire  is  0.04  sq.  inch.  What  is 
the  voltage  drop  in  the  main,  if  all  lamps  are  burning  simultane- 
ously ? 

Since  we  have  to  consider  both  the  positive  and  the  negative  main, 
the  total  length  of  wire  employed  will  be  160  yds.  As  is  generally 
the  case  for  mains,  the  wire  consists  of  copper,  whose  specific  resistance 
is  TQ  ire  o-  ohms  per  yard  per  sq.  inch;  the  total  resistance  of  the  mains 
is  thus  — 


The  current  required  for  feeding  20  16-candle-power  lamps  is  20  X^ 
=  10  amps. 

Thus  the  voltage  drop  in  this  main  —  which  may  be  called  e,  to 
distinguish  it  from  E  —  will  be  — 


To  get  the  proper  voltage  of  110  at  the  lamps,  we  want  in  the 
central  station,  say,  111  volts.  The  voltage  drop  in  the  main  is  thus 
not  quite  1  per  cent.,  which  may  be  allowed  in  any  case.  Even  if 
the  pressure  in  the  central  station  be  only  110  volts,  and  the  lamps 
therefore  burn  with  109  instead  of  110  volts,  this  would  not  be 
any  disadvantage,  as  the  diminution  of  the  light  is  not  serious  as 
long  as  the  voltage  falls  2  or  3  per  cent.  only. 

The  power  lost  in  the  main  is  1  voltXlO  amps.  =  10  watts;  or, 
using  the  formula  — 

P=C2XR=10X10XTV=10  watts. 

The  total  power  given  to  the  lamps  is  — 

EC  =  110X10  =1100  watts. 

Let  us  now  assume  that  we  have  to  transmit  through  a  main  of 
equal  cross-sectional  area  the  same  current  a  distance  of  800  yards 
(total  length  of  the  wire  =  1600).  Then  the  resistance  of  the  wire 
will  be  ten  times  as  large  as  in  the  above  example,  viz.  la>;  the  voltage 
drop  in  the  main  will  be  10,  the  power  loss  100  watts. 

These  losses  are  comparatively  very  great.  If  in  the  central 
station  a  voltage  of  110  be  maintained,  then  the  lamps  at  the  end  of  the 
main  would  burn  with  100  volts  only,  and  would  emit  far  less  light 
than  they  would  do  if  connected  with  their  proper  voltage;  further, 
the  loss  in  the  main  of  100  watts,  that  is  more  than  9  per  cent,  of 
the  total  output,  is  a  very  high  one. 


FUNDAMENTAL  PRINCIPLES 


45 


We  may  hence  lessen  the  voltage  and  the  power  loss  by 
diminishing  the  resistance  of  the  main,  i.e.  by  enlarging  the  cross- 
sectional  area  of  the  copper  wire. 

If  we,  for  instance,  quadruple  the  cross-sectional  area  of  the 
copper  wire,  then  its  resistance  becomes  the  fourth  part  only:  — 


and  then  the  voltage  drop  becomes  one-fourth  as  well  — 
e  =  HC  =  0.25X10  =  2.5  volts. 


The  power  lost  in  the  main  will  thus  be  — 
P  =  C2R  =  25  watts. 


These  values  of  voltage  drop  and  power  lost  are  allowable  in 
practice,  but,  as  we  have  seen  from  the  example,  we  get  these 
permissible  losses  only  by  employing  wires  having  large  cross- 
sectional  areas.  If  the  distance  were  still  longer  than  800  yards 
we  must  employ  wires  of  still  greater  areas,  and  so  the  network  of 
lines  would  become  exceedingly  costly. 

We  have,  however,  other  means  of  reducing  the  losses  due  to 
voltage  drop.  Suppose  we  double  the  voltage  in  the  central  station, 
and  connect  the  lamps  in  ten  parallel  groups,  each  of 
these  groups  consisting  of  two  series  connected  lamps 
(see  Fig.  43). 

The  resistance  of  each  of  these  groups  is  2X220  = 
44Qoj,  and  thus  the  current  taken  by  each  of  the 
groups,  if  connected  with  220  volts,  would  be  f}£=0.5 
amp.;  i.e.  the  same  as  taken  by  a  single  lamp  before. 
The  10  groups  together  require  thus  a  current  of 
10X0.5=5  amps. 

The  power  taken  by  the  20  lamps  is  obviously 
now  the  same  as  before.  It  was  110  volts  X  10  amps. 
=  1100  watts  in  our  first  example,  and  is  220X5  = 
1100  watts  in  this  one. 

For  the  mains  we  employ  the  same  wires  as  in 
the  first  example,  with  a  cross-sectional  area  of  0.04 
sq.  inches.  The  voltage  drop  in  this  main,  having 
a  resistance  of  \a),  is  5  volts  at  the  current  of  5  amps. 
These  5  volts  are  2.3  per  cent,  of  the  voltage  of  220 
volts,  thus  being  a  permissible  loss.  The  power  lost 
in  the  mains  is  52Xl  =  25,  i.e.  again  2.3  per  cent. 
of  the  total  load.  By  doubling  the  voltage  we  obtain, 
thus,  the  same  result  as  by  quadrupling  the  area  of  the 
cross-section. 


FIG.  43.  — 
Lamps  in 
Series  and 
Parallel. 


46  ELECTRICAL  ENGINEERING 

This  reasoning  explains  why  high  voltages  are  employed  when- 
ever electrical  energy  is  to  be  transmitted  long  distances.  A 
pressure  of  110  to  150  volts  is  generally  used  for  current  supplied  to 
a  single  building  only  or  to  several  buildings  situated  near  each 
other.  For  providing  small  districts  with  electrical  energy  a  pressure 
of  200  to  250,  and  for  larger  districts  400  to  SCO  volts  is  employed. 
But  even  these  voltages  are  not  sufficiently  high  for  mains  spread 
over  large  towns  and  districts.  To  get,  in  the  latter  cases,  allowable 
losses,  £nd  yet  not  have  too  large  a  size  of  mains,  voltages  of  1,000 
2,000,  5,000,  10,000  and  up  to  80,000  are  employed.  In  laying 
cables  for  such  high  voltages  special  care  has  to  be  taken  to  have 
good  insulation.  The  direct  connection  with  a  high-tension  line, 
or  even  through  any  substance  which  is  not  insulated  perfectly  from 
the  line,  may  have  a  fatal  effect.  . 

At  the  end  of  this  chapter  a  table  is  given,  showing  the  approxi- 
mate diameters  and  sectional  areas  of  the  wires  and  cables  mostly 
employed  in  practice.  Their  resistance  in  ohms  per  100  yards 
is  also  given.  By  means  of  this  table  we  can  calculate  the  sectional 
area  of  a  main,  if  its  length  and  the  current  be  given,  and  the 
voltage  drop  has  not  to  exceed  a  certain  amount.  This  problem 
has  to  be  solved  frequently  by  electrical  engineers. 

If  the  dimensions  of  all  lines  are  not  determined  before  laying 
them,  then  it  very  often  happens  that  the  voltage  drop  is  too  large, 
and  the  lamps  give  a  poor  light. 

Further  examples  of  installation  calculations  will  now  be  given. 

EXAMPLES. 

1.  A  group  of  ten  16-candle-power  110-volt  lamps  is  to  be  fed  by  means  of  a 
cable  whose  single  length  is  100  yards;  the  voltage  drop  is  not  to  exceed  about  2 
volts.     What  wire  should  be  employed? 

The  current  taken  by  the  10  lamps  is  5  amps.  The  voltage  drop  in  the  line  is 
€=CxR.  Hence,  since  the  current  C=5  amps,  and  the  voltage  drop  e=2  volts, 
the  resistance  R  of  the  line  must  be  f =  0.4o»,  or  less  if  a  smaller  voltage  drop  is 
taken.  The  length  of  the  lead  and  return  is  200  yards,  thus  the  resistance  per 
100  yards  of  the  wire  to  be  employed  must  not  exceed  0.4Xi$$=0.2w. 

As  we  see  from  our  table,  a  cable  of  7/18  S.W.G.  has  a  resistance  of  0.185o>  per 
100  yards.  This  cable  is  nearest  to  the  one  we  want,  whereas  the  resistance  of 
the  next  smaller  wire,  7/20,  is  0.329&>,  and  therefore  far  too  high.  We  shall 
prefer  to  employ  a  cable  of  7/18  S.W.G.  A  glance  at  the  table  shows  that  the 
maximum  current  allowed  for  this  cable  is  21  amps.,  giving  a  considerable  margin 
above  the  5  amps,  required  for  the  lamps. 

2.  A  current  of  30  amps,  at  a  voltage  of  250  is  to  be  conducted  as  far  as  300 
yards.     The  maximum  voltage  drop  allowed  is  3  per  cent,  of  the  total  voltage, 

i.e.  7.5  volts.     Then  the  resistance  of  the  line  may  be  -^-=0.25(0.     The  total 

length  of  the  cable  is  2X300=600  yards,  the  resistance  allowable  for  100  yards 
is  thus  0.25  X  J{$= 0.0416.  As  we  learn  from  the  table,  a  cable  of  19/16  S.W.G. 
has  to  be  employed  in  this  case. 


FUNDAMENTAL  PRINCIPLES  47 

3.  A  current  of  30  amps,  is  to  be  conducted  50  yards.  The  voltage  drop 
allowed  is  6  volts.  Then  the  resistance  allowed  for  the  total  length  of  wire,  viz. 
100  yards,  is  /^=0.2w.  From  the  table  we  learn  that  100  yards  of  a  cable  of 
7/18  S.W.G.  has  a  resistance  of  0.185w  only.  This  cable  would  therefore  be 
sufficiently  thick  with  regard  to  the  voltage  drop.  Notwithstanding,  we  must  not 
use  this  cable,  because  the  maximum  current  allowed  for  it  is  only  21  amps.  We 
must  therefore  take  the  nearest  cable  for  which  a  current  of  30  amps,  is  allowable, 
i.e.  19/20  S.W.G. 

Up  to  now  we  have  considered  in  our  calculations  the  voltage  drop 
and  the  power  loss  only.  But  another  very  important  point,  viz. 
the  heating  of  the  line,  must  not  be  neglected.  If  we  allowed  for  a 
short,  fine  wire  a  loss  equal  to  that  in  a  long,  thick  wire,  then  the 
former  would  be  far  more  heated  than  the  latter.  To  avoid  exces- 
sive heating  of  a  line,  the  current  strength  of  any  cable  has  not  to 
exceed  that  value  which  is  marked  in  our  table  as  "Maximum  Cur- 
rent Allowable,"  and  which  has  been  fixed  by  the  Institution  of 
Electrical  Engineers.  These  maximum  currents  have  been  selected 
so  that  the  rise  of  temperature  in  the  cables  will  be  about  20°  Fahr. 
above  the  surroundings. 

From  a  glance  at  the  table,  it  will  be  noticed  that  for  a  sectional 
area  of  0.0019  sq.  inch  the  maximum  current  is  4.4  amps.,  whereas 
for  a  sectional  area  of  0.0198  not  a  current  of  44,  but  only  of  30  amps., 
is  allowed.  For  0.19  sq.  inch  a  current  of  190  amps,  is  allowed, 
instead  of  440,  as  one  should  expect.  One  would  imagine  that  a 
cable  of  tenfold  sectional  area  could  also  safely  carry  the  tenfold 
current;  but,  as  a  matter  of  fact,  that  is  not  so.  The  temperature 
which  a  wire  attains  depends  on  the  rate  at  which  it  can  radiate  the 
heat  produced  in  it  to  the  surrounding  bodies.  To  explain  this  fact, 
let  us  consider  a  piece  of  copper  wire,  having  a  sectional  area  of 
0.0019  sq.  inch,  which  is  situated  in  the  centre  of  a  thick  wire  with  a 
sectional  area  of  0.019  sq.  inch.  Suppose,  now,  that  we  send  a  cur- 
rent of  44  amps,  through  the  thick  wire;  then,  in  the  centre-piece  of 
0.0019  sq.  inch  sectional  area  obviously  an  equal  quantity  of  heat 
will  be  produced  as  in  the  thin  wire,  having  0.0019  sq.  inch  cross- 
sectional  area.  But  the  heat  produced  in  the  centre  of  the  thick 
wire  cannot  be  led  away  as  quickly  as  with  the  thin  wire,  because  it 
has  to  go  through  the  whole  thickness  before  arriving  at  the  surface. 
Thus  a  wire  of  0.019  sq.  inch  sectional  area  carrying  44  amps,  would 
get  much  hotter  than  a  wire  of  0.0019  sq.  inch  area  carrying  4.4  amps. 
A  wire  with  0.19  sq.  inch  area,  carrying  440  amps.,  would  get  so  hot 
that  the  insulation  would  be  burnt  away  after  a  short  time. 

To  prevent  an  excessive  load  on  a  wire,  and  thus  its  dangerous 
heating,  a  fuse,  or  cut-out,  is  inserted  in  the  main,  the  whole  of  the 
current  therefore  flowing  through  it,  but  its  cross-sectional  area  is 
smaller  than  that  of  the  line  wire.  It  consists  of  an  easily  fusible 
metal,  such  as  lead,  tin,  or  alloys  of  them,  and  sometimes  of  silver 
and  copper. 


48 


ELECTRICAL  ENGINEERING 


With  the  normal  current,  with  which  the  beatings  of  the  mains  is 
hardly  appreciable,  the  fuse,  having  a  sectional  area  of  the  right  size, 
should  be  little  more  than  the  temperature  of  the  hand.  If  the 
current  doubles  in  strength,  then  the  heating  of  the  fuse  wire  should 
be  such  as  to  cause  it  to  melt.  By  this  means  the  main  current  is 
broken,  and  no  further  heating  can  occur. 

The  double  current  does  not  involve  any  danger  for  the  mains, 
for,  as  mentioned  above,  the  heat  produced  by  the  maximum  allow- 
able current  does  not  raise  the  temperature  of  the  wire  more  than  20° 
Fahr.  The  heat  now  produced  by  the  double  current  will  be  a  four- 
fold one,  the  rise  of  temperature  in  the  wire  will  thus  not  exceed 
80°  Fahr.  Assuming  a  room  temperature  of  85°  Fahr.,  the  temper- 
ature of  the  main  would  come  to  about  165°  Fahr.  All  kinds  of 
insulating  materials  used  for  mains  can  stand  this  temperature,  bufc 
a  higher  one  would  be  dangerous. 

Fuses  hence  furnish  an  excellent  means  of  preventing  dangers 
arising  from  electric  mains.  It  is,  of  course,  necessary  to  design 
fuses  so  that  they  cannot  give  rise  to  any  dangers  themselves  by 
melting.  They  must  be  fixed  on  an  incombustible  base — for  instance, 
marble  or  slate;  and  means  have  to  be  provided  to  prevent  melted 
metal  from  falling  on  inflammable  bodies.  Figs.  44  and  45  show 


FIG.  44. — Fuse  or  Cut-out 
(British  Schuckert  Co.). 


FIG.  45. — Fuse  or  Cut-out  for  Large 
Current  (British  Schuckert  Co.). 


two  designs  of  fuses,  or  cut-outs,  in  which  the  fusible  wire  is  within 
a  porcelain  handle,  enabling  the  cut-out  to  be  also  used  as  a  switch. 
In  Fig.  44  the  handle  is  intended  to  be  removed  directly,  but  in 
Fig.  45  it  may  be  hinged  back. 


FUNDAMENTAL   PRINCIPLES 


49 


Fig.  46  shows  a  form  of  fuse  used  extensively  in  America.  It 
consists  of  a  stout  tube  of  fibre  capped  at  the  ends  with  brass.  In 
this  tube  is  the  fuse,  made  of  copper  or  lead-antimony  alloy,  packed 
about  solidly  with  some  fire-proof  powder  like  lime  or  clay,  thus 
excluding  all  air.  The  ends  of  the  fuse  wire  are  fastened  to  the 
blade  terminals  of  the  fuse  which  project  inside  for  this  purpose. 
When  this  fuse  blows  there  is  no  sound  whatever  and  no  spark. 
An  ordinary  fuse  wire  when  it  melts  in  the  open  air  makes  a  loud 
noise  and  a  bad  spark,  particularly  when  inserted  in  a  circuit  of 
500  volts.  The  result  is  that  surrounding  parts  of  apparatus,  such 


FIG.  47.— 2300-volt  Expulsion  Fuse-block. 


FIG.  46.  — Enclosed 
Fuse. 


FIG.  48.— 2360-volt  Expulsion-tube 
Fuse-block. 


as  switchboards,  terminal  blocks,  etc.,  are  burned  and  injured  in 
appearance.  In  addition,  actual  injury  can  occur  to  individuals  if 
they  should  happen  to  be  near  at  the  time  of  the  melting.  Sparks 
may  fly  about  also  and  cause  an  actual  fire.  Thus  the  "  enclosed 
fuse  "  has  a  very  wide  use,  and  on  circuits  up  to  750  volts  and  cur- 
rents up  to  400  amperes  is  a  most  satisfactory  device  to  use. 

Figs.  47  and  48  show  two  important  types  of  fuse-block  suitable 
for  higher  voltages,  up  to  5000  volts.  Here  the  fuse  is  in  an  enclosed 
chamber  as  before,  but  it  is  not  packed  with  any  material.  Instead 
an  opening  is  purposely  left  to  the  open  air,  but  so  placed  that  the 
spark  resulting  from  the  interruption  of  the  circuit  from  the  melt- 
ing of  the  fuse  is  directed  in  a  proper  and  safe  direction.  The  prin- 
ciple of  this  fuse-block  is  that  the  gases  from  the  melted  fuse,  being 


50 


ELECTRICAL  ENGINEERING 


produced  suddenly,  "  snuff  out  "  the  arc,  the  vapors  shooting  out 
of  the  opening  at  the  same  time.  The  tube  type  is  the  more  effective. 
For  higher  currents  and  voltages,  a  device  called  a  circuit-breaker  is 
used  in  America.  Fig.  49  shows  one  of  them  built  for  600  volts 
and  300  amperes.  The  current  is  finally  broken  at  the  carbon  points 
shown  at  the  top  of  the  figure.  Carbon  has  the  ability  of  standing, 
without  particular  injury,  great  heat.  When  the  current  is  broken 
the  flash  and  the  arc  resulting  are  at  a  great  temperature.  Copper 
is  badly  injured  thereby,  sometimes  melting  in  drops.  The  current 


FIG.  49.— C.  P.  Circuit-breaker. 

therefore  is  carried  by  the  lower  contacts  in  the  figure,  but  the  break- 
ing is  done  at  the  carbon  contacts,  the  copper  and  carbon  being  in 
multiple,  but  the  carbon  leaving  last.  A  coil  as  shown  in  the  figure 
acts  as  an  electro-magnet.  If  the  current  gets  excessive  or  above 
a  certain  desired  point,  the  magnet  pulls  a  piece  of  iron  called  a 
keeper,  placed  in  front  of  it,  which  "  trips  "  the  breaker  just  as  a 
rat-trap  is  tripped. 

Fig.  50  shows  another  form  of  circuit-breaker  capable  of  break- 
ing 10,000  amperes  at  750  volts  without  being  injured  in  the  slightest. 
The  principle  upon  which  this  breaker  acts  is  different  from  the  other. 
Here,  in  addition  to  the  magnet  which  trips  the  breaker  when  the 
current  gets  strong  enough,  there  is  another  which  is  short-circuited 


FUNDAMENTAL  PRINCIPLES  51 

by  brushes  when  the  breaker  is  closed  and  carrying  current.     When 
the  breaker  trips  these  brushes  leave  their  contact  before  the  con- 


FIG.  50. — K  Breaker,  Large  Current. 

tacts  which  open  the  circuit  leave.  In  so  doing  they  therefore 
throw  the  current  into  the  magnet  which  they  short-circuited.  The 
magnet  is  placed  so  that  its  field  or  lines  of  force  pass  across  the  con- 
tact at  which  the  final  break  of  current  occurs.  The  result  is  that 
the  arc  resulting  from  the  break  is  " blown  out"  by  the  magnetism 
and  directed  up  a  shute  as  shown  in  the  figure,  thus  doing  no  harm.. 
This  blowing  out  of  the  arc  is  based  upon  Ampere's  rule;  the  cur- 
rent in  the  arc,  or  the  arc  itself,  being  deflected  up  by  the  magnetism. 

This  same  principle  of  blowing  out  arcs  by  magnetism  is  used 
in  controllers,  rheostats,  etc. 

A  properly  erected  electric  plant,  which  is  always  kept  in  order,  can 
hardly  ever  cause  any  danger  of  fire.  The  fuses  do  not  only  prevent 
a  permanent  overload  of  a  main  and  the  excessive  heating  connected 
with  it,  but  they  also  act  momentarily  when  a  short  circuit  takes 
place.  If,  for  example,  by  any  accident  the  positive  and  negative 
wires  be  connected  by  a  bare  metal  rod,  the  resistance  of  the  main 
becomes  so  small  that  a  very  great  current,  far  exceeding  double  the 
normal  current,  flows  through  the  line,  causing  the  fuses  to  melt  at 
once,  and  so  preventing  dangerous  heating. 


52  ELECTRICAL   ENGINEERING 

It  is  a  different  matter  with  badly  installed  plants  or  such  as  are 
not  kept  in  order.  If,  for  instance,  due  to  a  leakage  to  earth, 
the  current  flowing  through  the  main  is  greater  than  that  taken 
by  the  lamps,  then  a  frequent  melting  of  the  fuses  will,  of  course, 
happen.  If,  now,  the  person  in  charge  of  the  plant  or  a  thoughtless 
wireman  puts  thick  metal  strips  instead  of  those  of  normal  size  into 
the  fuses,  then  they  will  no  longer  melt.  This  is  just  LS  if  any 
one  tied  down  a  safety-valve  of  a  boiler  so  that  it  could  not  work 
when  the  pressure  is  excessive.  The  safety-valve  will  then  no  longer 
be  of  any  use,  and  the  boiler  may  burst.  A  similar  thing  may 
happen  with  electric  mains  when  thin  fuses  are  replaced  by  some 
of  too  great  sectional  area.  The  danger  is  especially  great  when 
increasing  earth-currents  are  no  longer  indicated  by  the  melting  of 
cut-outs.  Eventually  the  insulation  may  become  so  defective,  and 
the  earth-currents  so  strong,  that  the  mains  may  themselves 
actually  melt. 

It  is  not  absolutely  inadmissible  to  replace  thin  fuse-wires  by 
thicker  ones.  It  may  happen  sometimes  that  the  plant  has  been 
but  little  loaded  originally,  and  therefore  thin  fuse-wires  have  been 
used,  whereas  the  mains  would  have  been  able  to  carry  a  greater 
load.  If,  then,  another  number  of  lamps  be  connected  with  the 
mains,  the  replacement  of  the  thinner  fuse-wire  by  a  thicker  one  is, 
of  course,  allowable.  But  the  maximum  thickness  of  the  fuse-wire 
should  always  be  limited  by  the  fuse-current,  which  is  given  in  the 
table  for  the  copper  wires  of  different  cross-sectional  area  (see  Table 
on  next  page).  Thus  the  fuse-current  of  a  wire,  employed  for  a 
cable  of  No.  15  S.W.G.,  for  instance,  should  never  exceed  16.4  amps. 
What  current  is  necessary  to  melt  a  particular  piece  of  tin  or 
alloy  used  as  a  fuse-wire  is  best  obtained  by  experiment,  and  should 
be  noted  on  a  label  attached  to  the  bobbin  of  wire. 

The  circuit-breaker  being  designed  to  care  for  large  currents  is 
used   naturally  in   power  stations   and  on   switchboards   where   the 

energy  is  great.  They  are  used  in 
America  on  motor  installations  due 
to  the  ease  of  reclosing  the  circuit 
if  it  opens,  which  is  accomplished 
by  merely  closing  the  circuit- 
breaker  handle.  Fuses  are  used  usu- 
ally on  house  circuit.  The  most 
used  form  for  this  purpose  is  shown 
FIG.  51.— Plug  Fuse.  m  Fig.  51,  which  consists  of  a 

plug  just  like  a  lamp-socket  in  an 

enclosure  of  which  is  located  the  fuse.  Since  they  cost  so  little, 
they  are  thrown  away,  plug  and  all,  after  blowing,  being  replaced  by 
a  new  one;  thus  no  handling  of  the  fuse  proper  is  necessary. 


FUNDAMENTAL  PRINCIPLES 


53 


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FUNDAMENTAL  PRINCIPLES 


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CHAPTER   II 
MAGNETS— MAGNETIC  LINES  OF  FORCE 

Influence  of  a  Magnet  on  an  Electric  Current — 
Deprez  Instruments 

We  have  learned  that  a  freely  movable  magnetic  needle  is 
deflected  by  a  current  flowing  through  a  fixed  conductor.  If  we 
make  the  magnet  stationary  and  the  conductor 
movable,  we  shall  find  that  the  latter  will 
move  when  a  current  is  passed  through  it. 
This  can  be  well  observed  with  the  instrument 
shown  in  Fig.  52.  A  strong  magnetic  field  is 
produced  by  the  horseshoe  magnet  which  is 
provided  with  soft  iron  pole-pieces.  Within 
these  is  a  fixed  iron  cylinder.  By  the  action 
of  the  poles  of  the  magnet  the  cylinder  also 
becomes  a  magnet,  with  a  north  pole,  n,  and  a 
south  pole,  s.  In  the  air  gap  between  the 
magnet  poles  and  the  cylinder  a  coil,  consisting 
of  very  fine  wire,  is  arranged  so  as  to  be  easily 
movable.  The  ends  of  this  coil  are  connected 
by  means  of  very  fine  spiral  springs  with  the 
two  terminals  of  the  instrument.  These 
springs  hold  the  coil,  to  which  a  pointer  is 
attached,  at  a  position  of  rest,  and  at  the 
same  time  give  a  means  of  leading  a  current 
from  the  terminals  of  the  instrument  to  the 
coil.  Hence,  if  we  connect  the  terminals  of  the 
instrument  with  a  source  of  E.M.F.,  a  current  will  flow  through  the 
windings  of  the  coil.  It  flows,  for  instance,  in  the  left  part  of 'the 
windings  upwards,  and  in  the  parts  to  the  right  downwards.  Now 
let  us  consider  what  deflecting  actions  will  take  place  between  elec- 
tric current  and  magnet.  Imagine  the  swimmer  of  Ampere's  Rule 
in  the  part  of  the  coil  to  the  left  hand  of  the  reader.  On  this  part 
the  north  pole  of  the  magnet  and  the  south  pole  of  the  cylinder  are 
acting.  Let  the  swimmer  face  the  N  pole,  which  would  move  towards 
his  left  hand  if  it  were  not  fixed,  then  the  swimmer  being  himself 
movable  with  the  coil  will  be  urged  in  the  direction  of  the  arrow  1. 
The  effect  of  the  inner  s  pole  will  be  the  same,  because  we  must  now 
suppose  that  the  swimmer  is  facing  this  pole.  The  pole,  being  a 
south  one,  is  pressed  towards  his  right  hand,  but,  not  being  capable 
of  moving  again,  the  coil  must  be  driven  to  1,  just  as  if  his  right  hand 
were  pressing  in  such  a  way  as  to  move  him  along  the  face  of  the  s 
pole.  Next  considering  the  right-hand  part  of  the  coil,  the  swimmer 
must  proceed  with  his  head  turned  towards  the  paper  and  from  the 
reader;  then,  exactly  as  before,  we  shall  find  the  resultant  action  is  to 

56 


FIG.  52. — Deprez 
Instrument. 


MAGNETS— MAGNETIC  LINES  OF  FORCE 


57 


drive  the  coil  in  the  direction  of  the  arrow  2.  Thus  the  forces  acting 
tend  to  twist  the  coil  round  until  they  are  balanced  by  the  effort  of 
the  springs  to  drive  the  coil  in  the  opposite  direction. 

The  stronger  the  current  then  of  course  the  greater  will  be  the 
deflection,  so  that  if  a  pointer  be  fastened  to  the  coil,  arranged  so 
chat  it  moves  over  a  scale,  the  strength  of  the  current  can  be  inferred 
from  the  amount  of  the  deflection.  Hence  the  new  instrument  with 
which  we  have  become  acquainted  may  be  used  as  an  ammeter.  It 
is  called  after  the  inventor  a  Deprez  instrument. 

Important  details  of  one  make  of  a  "moving  coil  instrument"  (as 
it  is  often  called)  are  seen  in  Fig.  53.  These  instruments  have  very 


FIG.  53. — Construction  of  Moving  Coil  Ammeter 
(Weston  Electrical  Instrument  Co.). 


useful  properties.  If  the  current  be  reversed,  then  the  coil  will 
obviously  be  deflected  in  the  opposite  direction.  We  can  therefore 
furnish  the  instrument  with  a  scale  having  a  zero  at  the  middle,  and 
reading  both  to  the  right  and  to  the  left.  Then  the  pointer  gives  not 
only  a  measure  of  the  strength  of  the  current,  but  also  indicates  its 
direction.  If,  however,  the  instrument  is  furnished  with  cne  scale 
only,  reading,  say,  from  zero  to  the  right,  then  the  current  must 
always  be  sent  through  the  instrument  in  a  certain  direction.  The 
terminals  of  such  an  instrument  are  therefore  marked  +  and  — , 


58 


ELECTRICAL  ENGINEERING 


telling  us  that  the  leads  from  the  battery  must  be  connected  so 
that  the  positive  and  negative  poles  are  respectively  at  these  ter- 
minals. 

A  Deprez  instrument  may  be  used,  it  will  be  evident,  as  a  pole- 
finder.  If  the  deflection  is  along  the  scale,  then  we  know  that  the  + 
pole  is  connected  to  the  +  sign  on  the  instrument;  if  otherwise,  then 
the  —  sign  is  connected  to  the  +  pole. 

A  very  fine  wire  being  wound  on  the  coil,  the  instrument  can 
only  be  used  for  feeble  currents.     If  a  thick  wire  coil  were  used,  then 
it  might  be  serviceable  for  strong  currents,  but  such  an  instrument 
would  be  clumsy  and  not  sufficiently  sensitive.     It  is  quite  possible 
to  use  the  fine  wire  instrument  for  strong  currents  in  the  following 
way:   Suppose  that  a  coil  is  only  able  to  stand  a  current  of  T1Q-amp., 
but  that  we  had  to  measure  a  current  of  1  amp.     Then,  if  we  connect 
in  parallel  to  the  coil  of  the  instrument,  a  resistance,  called  a  shunt, 
through  which  nine  parts 
of  the  whole  current  flow, 
so  that  only  one  part  passes 
through  the  coil,  the  result 
will  be  that  the  shunt  will 
take   T9F   and   the   coil  T10- 
amp.     This   ratio   can  be 
obtained    by   making    the 
resistance  of  the  shunt  -^ 
of   the    resistance    of    the 
coil.     Thus   if   the   resist- 
ance of  the  coil  were  1&>, 
then   the   shunt   must  be 

|<w. 

The  same  instrument 
may  be  used  to  measure  a 
current  of  10  amps.,  if  we 
make  the  resistance  of  the 
shunt  =  T^W;  again,  if  a 
current  of  100  amps,  had 
to  be  measured,  we  should 
require  a  shunt  of  7i^,  and  so  on  for  still  greater  currents. 

The  Deprez  instrument  is  easily  adapted  as  a  voltmeter  by  con- 
necting a  sufficiently  large  resistance  in  series  with  the  coil.  If  we 
make  the  same  assumption  as  before,  and  take  as  the  maximum 
allowable  current  through  a  certain  instrument  to  be  TV  amp.,  and  its 
resistance  Ico,  then  it  follows  that  we  must  not  connect  the  instru- 
ment terminals  with  a  voltage  greater  than  -^.  For  measuring 
higher  pressures,  say  of  1  volt,  we  must  place  9&>  in  series  with  the 
instrument;  this  will  give  a  total  resistance  of  10,  and  a  current  of 
^  amp.  To  measure  10  volts  the  total  resistance  must  be  100<w,  so 


FIG.  54. — Weston  Ammeter  (Weston  Electrical 
Instrument  Co.). 


MAGNETS— MAGNETIC  LINES   OF  FORCE 


59 


that  the  extra  resistance  will  have  to  be  99&>.  In  the  same  way  a 
voltage  up  to  100  will  require  999&>,  and  a  voltage  up  to  1000  will 
require  9999&»  to  be  added. 

The  same  methods  are  applied  to  hot  wire  and  other  instruments 
in  order  to  measure  large  voltages  and  currents. 

In  the  case  of  voltages  and  currents  that  are  not  very  great,  the 
shunt  or  resistance  is  placed  within  the  instrument. 

For  technical  purposes  the  instrument  is  graduated  so  that  the 
pointer  directly  shows  the  current  or  voltage  of  the  circuit;  thus  in 
Fig.  54  an  illustration  of  an  ammeter  is  given  capable  of  measuring 
up  to  400  amps.  The  shunt  for  such  an  instrument  would  be  like 
that  of  Fig.  55,  which,  however,  is  only  for  150  amps.,  and  generally 


FIG.  55. — Shunt  for  Ammeter. 


is  made  of  a  number  of  strips  of 
manganin,  an  alloy  which  changes 
its  resistance  but  little  with  rise 
of  temperature. 

The  Weston  instruments,  used 
largely  in  the  United  States,  oper- 
ate under  the  same  principle  as 
the  Deprez.  The  General  Electric 
"astatic"  instruments  used  on 
switchboards  have  the  movable 
feature  similar  to  the  Deprez, 
but  instead  of  using  permanent 
magnets,  a  magnet  produced  by 
an  exciting  current  (i.e.,  an  elec- 
tro-magnet) is  used,  the  source  of 
excitation  being  usually  a  storage 
battery.  This  avoids  the  varia- 


FIG.   56.  —Astatic  instrument  for 
Switchboards. 


60  ELECTRICAL  ENGINEERING 

tion  which  may  occur  in  residual  or  remanent  magnetism.  A  picture 
of  such  an  instrument  is  shown  in  Fig.  56.  This  instrument  has 
a  lamp  behind  a  scale,  thus  illuminating  it. 


Influence  of  Electric  Currents  on  each  other— 
The  Electro-dynamometer 

We  know  that  between  an  electric  current  and  a  magnet  there  is 
an  action  capable  of  causing  motion.  This  follows  from  the  fact 
that  a  pole  tends  to  be  forced  along  a  "line  of  force,"  and  currents 
produce  lines  of  force.  For  instance,  a  helix  carrying  current  acts 
precisely  like  a  magnet,  being  influenced  by  magnets  as  well  as  by 
currents.  Hence  it  should  be  expected  that  two  currents,  each  pro- 
ducing lines  of  force,  would  act  upon  each  other.  To  prove  this  exper- 
imentally, use  a  fixed  coil,  consisting  of  a  number  of  windings  of  in- 
sulated copper  wire.  At  right  angles  to  this  fixed  coil  is  a  movable 
one.  A  current  can  be  passed  into  the  latter  by  means  of  wires 
which  dip  into  two  cups  containing  mercury,  as  will  be  seen  by  exam- 
ination of  Fig.  57.  Let  currents  be  passed  through  the  two  coils, 
when  it  will  be  found  that  the  outer  coil  will  be  deflected,  and  on 
reversing  one  of  the  currents — either  in  the  outer  or  inner  coil — the 
deflection  will  be  reversed.  Careful  tests  with  this  apparatus  will 
prove  that  when  the  direction  of  the  current  is  the  same  in  the  wires, 
that  is  to  say,  both  upwards^or^both  downwards,  then  attraction  results;  but 
when  the  direction  of  the  currents  is  opposed,  then  repulsion  takes  place. 

The  influence  of  electric  currents  on  each  other  is  called  electro- 
dynamic  action.  The  word  dynamic  is  derived  from  the  Greek  word 
dynamis,  meaning  force. 

The  arrangement  just  described  may  be  used  for  the  purpose  of 
measuring  current  strengths.  Instruments  of  this  type  are  called 
electro-dynamometers,  and  usually  have  a  pointer  attached  to  the 
movable  coil.  A  wattmeter  of  this  type  will  be  described  in  later  pages. 

An  interesting  and  valuable  rule  results  from  the  action  described 
in  Fig.  57.  Let  it  be  supposed  for  a  moment  that  the  coil  B  is 
stationary  and  carrying  current  as  shown  by  the  arrows.  Let  it  be 
supposed  that  the  coil  A,  carrying  current  as  shown  by  arrows,  can 
freely  move. '  According  to  the  rule  just  explained,  the  side  C  of  coil 
A  is  attracted  to  side  D  t^coil  B;  or,  in  other  words,  the  coils  tend 
to  lie  in  the  same  plane.  'Consider  the  lines  of  force  created  by  coil 
B;  they  come  up  out  of  the  paper  inside  of  the  coil.  If  the  coil  A 
lies  flat  with  B,  it  then  contains  all  of  the  lines  of  force  that  it  can. 
In  other  words,  a  coil  free  to  move  under  the  action  of  electro-dynamic 
force,  tends  to  move  so  as  to  include  the  maximum  number  of  lines  of  force. 

If  the  current  be  reversed  in  A,  the  coil  would  tend  to  present  its 


MAGNETS— MAGNETIC   LINES  OF  FORCE 


61 


other  face  to  the  reader,  still  following  the  above  rule  in  so  doing. 
In  considering  this  valuable  rule,  remember  that  the  lines  of  force 


D 


J 
FIG.  57,— An  Electro-dynamometer. 

in  one  direction  count  positive  (in  the  above  figure,  those  coming  up 
out  of  the  paper),  and  in  the  other  negative. 

Electro-magnets 

Our  early  experiments  have  taught  us  that  a  piece  of  soft  non- 
magnetic iron  placed  within  a  coil  becomes  magnetic  as  soon  as  a 
current  is  sent  through  the  coil.  Such  an  arrangement  is  called  an 
electro-magnet.  The  iron  may  have  any  shape;  it  is  generally  in 
the  form  of  a  bar  or  horseshoe. 

How  the  iron  acquires  its  magnetism  is  not  very  easily  explained, 
but  a  comparison  will  enable  us  to  understand  what  probably  takes 
place.  Suppose  that,  instead  of  the  iron  bar,  we  had  a  tube  filled 
with  a  great  number  of  exceedingly  small  magnetic  needles.  On 
shaking  the  tube  the  needles  will  so  set  themselves  that  the  tube  shows 
no  or  nearly  no  apparent  magnetism.  Place  this  tube  within  a  coil 
through  which  a  current  is  passed.  A  directing  action  will  now  be 
exerted  on  each  of  the  magnetic  needles,  and  they  will  attempt  to 
turn  in  a  certain  direction.  But  the  magnetic  needles  not  being  freely 
movable,  hence  offer  a  certain  resistance  to  their  rotation.  If  the 
current,  and  therefore  the  directing  force  exerted  on  the  needles,  is 
but  small,  then  the  resistance  will  prevent  the  needles  from  entirely 
following  the  directing  force. 


•62  ELECTRICAL  ENGINEERING 

A  certain  amount  of  rotation  of,  at  least,  the  easier  movable 
needles  will,  however,  take  place.  The  poles  of  the  needles  will  no 
longer  be  arranged  in  a  confused  manner,  but  their  north  poles  are 
directed  more  or  less  towards  one  end,  say  to  the  right,  the  south 
poles  more  or  less  towards  the  left.  The  tubs  will  now  be  magnetic. 
If  we  strengthen  the  current  flowing  through  the  coil,  its  directing 
force  on  the  needles  becomes  a  greater  one,  and  with  a  very  large 
current  the  directing  force  may  overcome  the  resistance  to  motion 
entirely,  and  all  the  needles  group  themselves  in  the  direction  of  the 
lines  of  magnetic  force  of  the  solenoid.  The  tube  now  shows  very- 
strong  magnetism.  If  the  current  flows  around  the  coil,  as  shown  in 
Fig.  20,  a  north  pole  will  be  formed  to  the  right,  and  a  south  pole  to 
the  left  of  the  tube.  If  Fig.  20  be  considered  attentively,  we  are  able 
to  deduce  the  following  rule: 

At  that  end  at  which  (looking  towards  this  end)  the  current  flows 
in  a  direction  which  is  counter-clockwise  round  the  coil  there  is  a 
north  pole;  and  at  that  end  at  which  (looking  towards  this  end)  the 
current  flows  clockwise  round  a  coil  a  south  pole  is  formed. 

From  many  other  phenomena,  not  only  of  a  magnetic  nature, 
which  could  not  have  been  otherwise  explained,  it  has  been  con- 
cluded that  the  smallest  parts  of  which  a  body  consists  are  not 
joined  together  rigidly,  but  possess  a  certain  mobility.  These 
smallest  parts,  which  cannot  be  made  smaller  by  mechanical  means, 
are  called  molecules. 

We  have  now  to  imagine  that  every  molecule  of  the  iron  is  a 
diminutive  magnet.  If  these,  which  we  may  call  molecular  magnets, 
lie  in  confusion,  then  the  iron  bar  will  be  like  that  within  the  tube 
of  the  previous  page,  and  have  no  apparent  magnetism;  but  if  we 
exert  a  directive  action  on  the  iron,  by,  for  example,  bringing  near 
to  it  the  north  pole  of  a  strong  magnet,  then  all  the  south  poles  of  the 
molecular  magnets  will  turn  towards  the  strong  north  pole  and  the 
north  poles  in  the  opposite  direction,  and  the  iron  will  now  be 
magnetized.  If  the  cause  of  the  directing  force  be  removed,  then  the 
molecular  magnets  return  to  their  original  position,  either  partially  or 
entirely. 

We  say  partially  or  entirely,  for  if  the  molecular  magnets  are 
difficult  to  move,  a  great  force  will  be  necessary  to  deflect  them  from 
the  position  of  rest.  If,  then,  the  deflecting  force  ceases,  the  particles 
will  not  return  to  their  original  position  because  the  resistance  which 
opposed  the  first  motion  will  also  resist  any  retrograde  action.  Iron 
of  this  kind  will  show  magnetic  properties,  even  after  the  magnetizing 
force  ceases.  Such  magnetism  is  called  residual  magnetism  or 
remanence. 

Iron  having  molecules  that  are  easy  to  turn  will  be  easy  t3 
magnetize.,  and  will  readily  return  to  the  non-magnetic  condition. 

Iron  shows  a   different   degree   of  remanence,   according   to   its 


MAGNETS— MAGNETIC  LINES  OF  FORCE  63 

hardness;  hence  it  is  rather  difficult  to  magnetize  hard  steel,  but  its 
residual  magnetism  is  of  a  large  amount. 

Very  soft,  especially  annealed  wrought  iron  can  be  magnetized 
very  easily  and  strongly,  and  in  a  far  higher  degree  than  steel;  its 
remanence  is,  on  the  other  hand,  small,  and  very  much  less  than  that 
of  hard  steel. 

With  steel  we  generally  do  not  speak  about  a  residual,  but  rather 
of  a  permanent  magnetism. 

It  follows  from  the  molecular  theory  of  magnetism  that,  if  we 
break  a  magnet  into  two  parts,  we  cannot  have  one  half  containing 
north,  and  the  other  half  containing  south  magnetism  only.  Even 
if  we  divide  a  magnet  into  exceedingly  small  parts,  each  of  these  will 
have  both  a  north  and  a  south  pole. 

A  field  of  magnetic  force  exists  both  in  the  space  outside  a 
magnet  and  also  in  its  interior;  the  lines  are  supposed  to  pass  from 
a  north  pole  to  a  south  pole  in  the  external  field,  and  then  to  travel 
through  the  magnet  from  south  to  north  pole,  forming  what  is  called 
a  magnetic  circuit.  This  magnetic  circuit  is  very  analogous  to  the 
electric  one. 

In  our  future  discussions  about  magnets  electro-magnets  will  be 
chiefly  considered,  as  these  are  of  much  greater  technical  interest 
than  permanent  ones. 

The  exciting  power  of  magnetism  or  magneto-motive  force  is  rep- 
resented by  the  effect  of  the  current  flowing  through  the  solenoid.  As 
wre  are  aware,  this  effect  depends  merely  on  the  strength  of  the  current 
and  the  number  of  turns.  Hence  the  number  of  ampere-turns  is  a 
measure  of  the  magneto-motive  force.  The  greater  the  latter  is  the 
larger  is  the  number  of  lines  of  force,  and  the  stronger  the  magnetic 
flux.  But  this  flux  depends  not  only  on  the  exciting  force,  but  also 
on  the  resistance  which  is  opposed  to  the  passage  of  the  lines  of 
force.  This  resistance  is  quite  analogous  to  the  electric  resistance, 
and  depends  on  the  length  of  the  path,  the  cross-section,  and  the  kind 
of  material. 

Iron  offers  a  low  resistance  to  the  lines,  whereas  air  and  all  non- 
magnetic materials  have  a  far  higher  resistance.  Hence,  if  we  wish 
a  strong  magnetic  flux,  we  must  make  the  path  through  the  bad 
conductor  (generally  air)  as  short,  and  its  sectional  area  as  great,  as 
possible. 

We  shall  now  be  able  to  understand  why  a  horseshoe  magnet 
exerts  a  far  stronger  force  than  a  bar  magnet  of  equal  strength  of  pole. 
If  we  bring  the  piece  of  iron  called  the  keeper  near  a  horseshoe 
magnet  (see  Fig.  58),  the  lines  of  force  pass  only  a  short  distance 
through  the  air.  The  greater  part  of  their  path  is  through  iron, 
either  that  of  the  magnet  or  of  its  keeper.  Since  the  path  through 
the  keeper  has  a  lower  magnetic  resistance  than  that  through  the  air, 
nearly  all  the  lines  will  go  through  the  keeper,  and  only  a  com- 


64 


ELECTRICAL   ENGINEERING 


paratively  small  number  of  them  will  take  other  paths  through  the 
air.  These  last-mentioned  lines  are  called  stray  lines.  They  are  of 
no  utility,  for  only  those  lines  which  reach  the  keeper  can  cause 
attraction. 

If  now  we  bring  the  same  keeper,  as  in  the  previous  example,  to 
the  pole  of  a  bar  magnet,  then  we  observe  that  all  the  lines  have  an 


FIG.    58. — Horseshoe  Electro- 
magnet 


FIG.    59. — Straight  Electro- 
magnet. 


air  path  (see  Fig.  59),  and  very  few  pass  through  the  keeper,  so  that 
the  useful  lines  are  very  few. 

Owing  to  the  long  air  path  the  magnetic  circuit  here  has  a  very 
high  resistance,  and  for  the  same  magneto-motive  force  the  number 
of  lines  will  be  far  less  than  in  the  case  of  the  horseshoe  magnet. 
The  flux  into  the  keeper  being  small,  the  pull  upon  it  by  the  magnet 
will  be  correspondingly  small. 

The  law  connecting  the  flux  with  the  exciting  power  and  resistance 
is  known  as  Ohm's  Law  for  magnetism.  There  is,  however,  a  very 
important  difference  between  this  law  and  the  Ohm's  Law  for  the 


INDUCTION 


65 


electric  circuit.  In  the  case  of  the  electrical  current  a  double  E.M.F. 
will  cause  a  double  current,  and  a  pressure  one  hundred-fold  will 
give  a  current  one  hundred  times  as  great  through  any  constant 
resistance.  With  the  magnetic  circuit  this  is  by  no  means  the  case. 
As  we  understood  from  the  discussion  about  magnets,  there  is  a 
defined  limit  above  which  the  magnetism  cannot  be  further  increased. 
This  maximum  is  reached  when  all  the  molecular  magnets  are  pulled 
into  a  straight  line.  Hence,  as  we  approach  this  condition  of  satu- 
ration any  increase  of  magneto-motive  force  is  practically  useless. 
Further,  the  magnetic  resistance  of  such  saturated  iron  is  very 
great. 

Nevertheless,  up  to  a  certain  point  the  magnetic  is  like  the 
electric  circuit.  A  great  increase  of  the  electric  pressure  produces 
a  very  strong  current,  which  heats  the  wire  and  causes  it  to  have  a 
higher  resistance  than  before,  preventing  therefore  the  current  from 
becoming  so  great  as  it  would  be  if  the  wire  had  been  kept  at  the 
original  temperature.  Compared  with  the  corresponding  increase 
of  the  magnetic  resistance  this  change  of  electric  resistance  is  small. 

Induction 

We  have  learnt  that  an  electric  current  flowing  through  a  con- 
ductor in  a  magnetic  field  is  capable  of  producing  motion  of  the  con- 
ductor or  of  the  magnet.  From  the  law  of  production  of  electro-motive 
force  by  the  cutting  of  lines  of  force,  a  volt  being  produced  by  the  cutting 
of  100,000,000  lines  of  force  per  second, 
it  follows  that  in  a  conductor  which  is 
made  to  move  in  a  magnetic  field 
an  electric  current  is  produced. 

To  prove  this,  let  the  following 
experiment  be  tried:  In  front  of  the 
poles  of  a  horseshoe  magnet  move  a 
copper  rod,  which  has  its  ends  con- 
nected by  flexible  wires  to  a  sensitive 
Deprez  ammeter,  as  shown  in  Fig.  60. 
If  we  move  the  copper  rod  in  any 
direction,  say  from  left  to  right  above 
the  north  pole,  the  ammeter  will  show 
a  sudden  deflection.  If  we  move  the 
rod  in  the  same  way  above  the  south 
pole/  the  deflection  will  be  in  an 
opposite  direction.  If  now  the  rod  be  moved  from  right  to  left 
the  deflections  will  be  opposite  to  the  corresponding  ones  of  the  first 
direction  of  motion  for  each  pole.  As  the  result  of  the  motion,  we 
therefore  produce  an  E.M.F. ,  whose  direction  depends  both  on  the  way 
that  the  lines  of  force  proceed  through  the  conductor,  and  on  the  way 


FIG.  60. — A  Conductor  in  a 
Magnetic  Field. 


C6  ELECTRICAL   ENGINEERING 

the  conductor  is  moved.  On  stopping  the  movement  of  the  conductor 
the  needle  of  the  ammeter  immediately  comes  to  rest,  proving  that 
motion  is  essential  for  the  maintenance  of  the  generated  electrical 
pressure.  We  shall  further  find  that  it  is  a  matter  of  indifference 
whether  we  move  the  conductor  or  the  field. 

If  there  is  no  closed  circuit,  a  current  cannot  of  course  be  pro- 
duced, but  an  E.M.F.  will  exist  immediately  the  conductor  moves; 
just  as,  hi  the  case  of  a  galvanic  cell,  an  E.M.F.  is  present  even  if 
the  poles  are  not  connected. 

It  is  of  practical  importance  to  determine  in  all  cases  the  direction 
of  the  current  in  the  moving  rod. 

It  will  have  been  remarked  that,  in  Nature,  whenever  a  motion 
takes  place  there  exists  some  resistance  which  tries  to  bring  the 
moving  body  to  rest.  To  overcome  this  resistance  work  has  to  be 
done.  The  ground,  for  example,  offers  a  resistance  to  the  movement 
of  a  vehicle.  Such  resistance  is  known  as  friction.  If  the  moving 

force  be  withdrawn,  friction  will 
gradually  cause  the  vehicle  to 
stop.  It  is  exactly  as  if  there 
existed  a  force  which  acts  in  a 
direction  opposite  to  that  of  the 

FIG.  61. -Action  of  Moving  Boat.       motion.     If   a    boat   is    moved 

on    water    (see    Fig.    61),    the 

water  is  raised  in  front  of  the  boat,  which  will  try  to  drive  the  boat 
in  the  opposite  direction,  and  will  really  do  so  as  soon  as  the  moving 
force  ceases. 

It  is  precisely  the  same  with  the  moving  conductor.  As  it  is 
made  to  travel  in  the  magnetic  field  a  current  is 
produced  in  such  a  direction  as  to  oppose  the 
motion  of  the  conductor.  Ampere's  Law  will  help 
us  here.  An  experiment  (Fig.  62)  will  show  that  if 
the  conductor  be  moved  to  the  right  a  current 
will  be  produced  so  as  to  flow  towards  the  spec- 
tator (this  direction  is  indicated  by  a  dot  in  the 
diagrams.  Such  being  the  direction  of  the  current 
FIG.  62.  -  the  swimmer  in  the  current  may  be  thought  of  as 
Direction  pushing  along  the  face  of  the  fixed  N  pole  with  his 
left  hanc*'  anc*  hence  he  tends  to  drive  the  conductor 
towards  his  right  hand  in  the  direction  of  the  feathered 
arrow. 

To  overcome  this  backward  force  of  the  current  we  must  do 
work  to  move  the  conductor  .in  the  intended  direction.  The  larger 
the  produced  current — and  we  may  alter  this  according  to  the  re- 
sistance connected  with  the  outer  circuit — the  larger  will  be  the 
retarding  force,  and  thus  the  greater  must  be  the  work  which  we 
have  to  exercise  to  move  the  conductor.  Hence  it  follows  that  we 


ELECTRICAL   MACHINES 


FIG.  63.— The  Hand 
Rule. 


do  not  get  the  current  for  nothing,  but  we  must  employ  a  certain 
amount  of  mechanical  effort.  We  therefore  only  transform  mechan- 
ical into  electrical  energy. 

A  rule  will  now  be  given  by  the  aid  of  which  it  is  possible  to 
determine  the  direction  of  the  produced,  or.  as  it  is  called,  the  induced 
current  in  a  much  simpler  way  than  employing 
Ampere's  Law  each  time. 

Hold  the  palm  of  the  right  hand  against  the 
lines  of  force,  the  thumb  in  the  direction  of  the 
motion,  then  the  fingers  point  out  the  direction  of 
the  induced  current  (see  Fig.  63). 

In  the  case  of  the  example  of  Fig.  62  the 
palm  of  the  hand  would  have  to  be  turned 
downwards,  against  the  north  pole,  since  the 
lines  of  force  proceed  upwards.  On  then 
holding  the  thumb  to  the  right,  the  fingers 
point  towards  the  spectator,  indicating  the 
direction  of  the  current  as  proved  by  ex- 
periment. A  very  useful  rule  to  bear  in  mind  as  to  direction  of 
induced  currents,  of  course  dependent  upon  the  same  principles  laid 
down,  ^s  looking  at  an  electric  circuit  in  the  direction  of  the  lines  of 
force  (i.e.,  in  the  direction  a  free  north  pole  would  tend  to  go),  if  the 
lines  of  force  are  increasing,  a  current  tends  to  flow  in  a  counter-clock- 
wise direction.  If  decreasing,  in  a  clockwise  direction. 

There  is  another  and  important  case  of  induction  that  mu^t 
be  studied.  If  we  wind  a  wire  round  a  core  of  soft  iron,  and 
connect  its  ends  with  the  terminals  of  a  Deprez  ammeter,  we  can 
observe  a  deflection  on  the  instrument  immediately  we  approach  a 
magnet  pole  to  the  core.  When  the  magnet  comes  to  rest  the 
deflection  at  once  ceases.  If  we  take  away  the  magnet  from  the 
core  a  deflection  is  produced  in  the  opposite  direction. 

The  same  effects  can  be  observed  by  winding  another  coil  on  the 
core  which  is  connected  with  some  source  of  E.M.F.  As  long  as 
the  current  in  the  new  coil  remains  constant — that  is  to  say,  as  long 
as  the  flux  does  not  alter — we  cannot  observe  any  current.  But  as 
soon  as  we  strengthen  or  weaken,  start  or  stop  the  magnetizing 
current  we  get  a  deflection  of  the  ammeter  which  is  greater  the 
greater  is  the  variation  of  the  magnetizing  current,  and  the  more 
rapidly  the  alteration  is  caused. 

Thus  an  E.M.F.  is  always  induced  in  a  winding  surrounding 
an  iron  core  if  the  magnetism  of  the  core  is  either  strengthened  or 
weakened. 

This  law  that  the  induced  current  is  in  such  a  direction  that  it 
tends  to  stop  the  motion,  as  described  in  connection  with  Fig.  62, 
is  covered  by  what  is  now  generally  known  as  Lenz's  law,  which  is. 
that  "in  all  cases  of  magnetic  induction  the  induced  currents  are  in. 


68 


ELECTRICAL  ENGINEERING 


such  a  direction  that  their  reaction  tends  to  stop  the  motion  that  produced 
them." 


Electrical  Machines 


If  we  could  by  any  special  device  move  a  conductor  repeatedly 
backwards  and  forwards  in  front  of  a  pole  of  a  magnet,  we  should 
obtain  a  current  which  would  change  its  direction  with  each  alteration 
of  the  direction  of  motion.  This  would  be  the  simplest  form  of  an 
electrical  machine  serving  for  the  transformation  of  mechanical  into 
electrical  energy. 

A  motion  backwards  or  forwards,  or  up  and  down,  is  called  a 
reciprocating  motion,  and  is  generally  avoided  from  a  mechanical 
point  of  view.  A  rotating  motion  is  much 
more  preferable  and,  as  it  is  easy  to  construct 
an  electrical  generator  or  dynamo  with  con- 
ductors which  rotate,  this  is  the  usual  method 
of  construction. 

In  Fig.  64   is  shown  a  horseshoe  magnet, 
which  is  similar  to  that  of  a  Deprez  instrument, 
and  is  provided  with  pole-shoes  of  soft  iron 
having  a  circular  bore.     To  give  the  lines  of 
magnetic  force  a  very  short  air  path,  in  the 
Deprez   instruments   a   fixed   iron    cylinder  is 
placed  inside  the  circular  space.     Within  the 
air  gap  the  conductors  are  movable.     The  same 
device  would  serve  as  a  dynamo  too,  but  the 
rotation  of  a  fine  wire  coil  in  so  small  a  space 
is  not  a  very  practical  construction,  and  would 
be  far  too  fragile  for  a  dynamo.     A  more  satis- 
factory way  is  to  fix  the  wire  to  the  iron  cylinder, 
and  make  them  revolve  together.     To  this  part 
the  name  armature  is  given. 
One  method  of  making  an  armature  is  shown  in  Fig.  64,  which  is 
called,  after  its  inventor,  a  Siemens  armature,  or,  after  its  method  of 
construction,  a  shuttle  or  H-shaped  armature.     It  will  be  seen  that 
the  iron  cylinder  has  two  slots  in  which  the  wire  is  wound. 

The  effect  of  the  winding  in  cutting  through  the  lines  of  force  is 
the  same  as  in  the  case  of  the  Deprez  construction;  for,  the  iron 
armature  is  not  a  permanent  magnet,  but  serves  only  for  transmitting 
the  lines  of  force  from  the  north  to  the  south  pole.  Whether  the 
armature  is  rotating  or  not,  the  lines  of  force  always  keep  in  the 


N 


FIG.   64. — A  Magneto- 
Generator. 


ELECTRICAL  MACHINES. 


same  direction.  They  do  not  rotate  together  with  the  armature,  but 
always  flow  in  a  horizontal  direction  from  the  north  to  the  south  pole. 
The  direction  of  the  current  produced  in  the  windings  we  may 
easily  determine  by  means  of  the  various  rules  presented.  Take,  for 
instance,  the  hand  rule.  Let  us  assume  the  rotation  of  the  armature 
to  be  clockwise.  If  we  consider  now  those  wires  which  pass  the  north 
pole  at  this  moment,  then  we  have  to  hold  the  palm  towards  the 
north  pole  (i.e.  towards  the  left)  and  the  thumb  in  the  direction  of 
motion,  or  upwards.  The  fingers  are  then  directed  behind  the  plane 
of  the  drawing.  Hence  in  all  wires  to  the  left  a  current  will  be  pro- 
duced which  flows  from  the  spectator.  (Marked,  in  Fig.  65,  by 
crosses.)  We  have  now  to  consider  the  wires  to  the  right,  which  are 
near  the  south  pole.  The  thumb  in  this  case  must  be  held  down- 
wards, because  the  armature  with  the  wires  moves  downwards  on 
this  side  also,  and  the  palm  must  be  turned  towards  the  north  pole 
as  before.  The  fingers  point  towards  the  spectator,  this  direction  of 
the  current  being  indicated,  in  Fig.  65,  by  dots  within  the  circles 
representing  the  wires.  From  the  same  diagram,  which 
shows  also  a  plan  of  the  windings,  we  learn  that  all 
the  induced  E.M.F.'s  add  themseives.  If,  for  instance, 
there  be  6  windings  or  12  conductors  on  the  armature, 
the  total  E.M.F.  produced  in  the  latter  will  be  12  times 
that  induced  in  a  single  wire.  If  we  wind  1000  turns 
of  a  very  fine  wire  on  the  armature,  we  may  therefore 
get  a  considerable  voltage,  especially  if  there  is  any 
arrangement  to  make  the  armature  rotate  very  quickly. 
This  may,  for  instance,  be  accomplished  by  a  suitable 
toothed  wheel  gearing. 

Consider  the  production  of  electro-motive  force  by 
the  rule  that  looking  along  the  lines  of  force  if  the  flux 
in  the  circuit  is  increasing,  the  electro-motive  force  is 
induced  which  tends  to  produce  a  current  in  a  counter- 
clockwise direction.  We  must,  in  Fig.  64,  look  from 
the  north  to  the  south  pole,  for  that  is  the  direction 
of  the  lines  of  force.  In  the  position  shown  in  the 
figure  the  armature  coil  is  containing  no  lines  of  force, 
being  edgewise  to  them.  As  it  turns  clockwise  it 
commences  to  take  lines;  hence  the  lines  are  increasing,  and  a  cur- 
rent tends  to  flow  counter-clockwise,  or  from  the  spectator  on  the 
left  and  toward  on  the  right. 

To  connect  the  armature  with  the  outer  circuit,  we  fix  each  of  the 
ends  of  the  coil  to  a  copper  or  brass  slip-ring  (see  Fig.  66). 

On  these  rings,  metal  springs  or  brushes  press  which  may  be 
connected  with  the  outer  circuit.  For  it  is,  of  course,  impossible  to 
connect  the  wires  of  the  outer  circuit  directly  with  the  ends  of  the 
coil  and  yet  permit  free  rotation. 


FIG.  65.— The 
Siemens  Ar- 
mature. 


70 


ELECTRICAL  ENGINEERING 


After  the  armature  has  made  a  quarter  of  a  revolution  we  observe 
that  the  wires  are  neither  within  the  influence  of  the  north  nor  of  the 
south  pole.  They  are  exactly  as  far  from  the 
north  as  from  the  south  pole.  Therefore  at 
this  moment  no  E.M.F.  at  all  is  induced  in 
them.  When  the  armature  passes  from  this 
position,  the  wires,  which  before  have  been 
embraced  by  the  north  pole,  come  now  to 
the  south  pole,  and  viceversd,  thus  the  direction 
of  the  induced  E.M.F.  is  altered.  The  current, 
flowing  through  the  outer  connection,  hence 
alters  its  direction  at  each  half  revolution  of 
FIG.  .66.— Slip-Rings.  the  armature. 

At  the  position  of  the  armature  shown  in  Fig.  64,  the  current  has 
its  maximum  value,  for  the  lines  of  force  are  here  being  cut  at 
the  maximum  rate.  Then  it  decreases  gradually,  and  becomes  nil 
after  a  quarter  revolution  of  the  armature,  and  then  gradually  grows 
to  a  maximum  (but  in  a  reversed  direction),  becomes  again  nil,  and  so 
on.  We  shall  be  able  to  understand  these  changes  better  by  drawing 
a  wavy  line,  such  as  shown  in  Fig.  67.  A  point,  moving  on  this 
wave  line,  has  at  a  defined  time  its  highest  position,  marked  in  the 
figure  by  a;  its  height  decreases  then  gradually,  and  becomes  zero 
at  6.  Then  the  point  descent^  beneath  the  horizontal  line,  till  it 
reaches  its  lowest  position  at  c,  which  is  exactly  as  far  under  the 


FIG.  67. — Alternating-Current  Curve. 


horizontal  line  as  the  point  a  is  over  the  horizontal  line.  The  point 
now  ascends  until  it  reaches  at  d  the  horizontal  line,  and  continues 
to  rise  until  at  e  its  highest  position  is  reached,  which  is  equal  to  ths  t 
of  a.  From  here  the  previous  changes  are  repeated. 

The  current  thus  generated  is  quite  different  to  that  taken  from 
a  galvanic  cell,  which  is  constant  in  strength  and  direction  as  long 


ELECTRICAL   MACHINES 


71 


FIG.  68. — Simple  Commutator. 


as  we  do  not  alter  the  resistance  of  the  circuit  or  the  connection  of 
the  poles,  and  is  therefore  called  a  constant  current.  The  current, 
on  the  other  hand,  taken  from  the  armature  just  described,  is  called 
an  alternating  current,  and  each  up-and-down  change  of  the  current, 
as  from  b  through  c  to  d  (Fig.  67),  is  called  an  alternation.  If  the 

armature  of  such  a  two-pole  or  bipolar 
dynamo  makes  1000  revolutions  per 
minute,  then  the  number  of  alternations 
in  the  same  time  is  2000. 

For  certain  purposes  the  employ- 
ment of  alternating  currents  is  of  great 
advantage.  It  is,  however,  very  often 
desirable  to  obtain  a  rectified  or  con- 
tinuous current.  If  the  armature  be 
rotated  very  slowly,  such  a  current  may 
be  obtained  by  changing  the  wires  going 
to  the  slip-rings  after  each  half  revolu- 
tion, at  the  moment  the  current  is 
reversing  its  direction.  Changing  of 
the  wires  by  hand  is  naturally  im- 
possible at  the  usual  speed  of  rotation.  A  " commutator"  enables 
this  difficulty  to  be  readily  overcome.  It  consists  (see  Fig.  68)  of 
two  half  rings,  which  are  insulated  from  each  other.  One  of  these 
half  rings  is  connected  with  the  beginning,  the  other  with  the  end  of 
the  armature  coil.  The  brushes  are  opposite  each  other,  one  on  the 
highest,  the  other  on  the  lowest  point  of  the  split  ring.  With  the 
brushes  1  and  2  the  outer  circuit  is  connected.  The  position  of  the 
commutator  shown  in  Fig.  68  corresponds  with  the  armature  position 
of  Fig.  64.  Let  the  commutator  revolve  in  the 
direction  of  the  arrow.  Then  Fig.  69  will  show 
its  position  a  quarter  of  a  revolution  afterwards, 
and,  until  reaching  this  position,  an  E.M.F.  will 
have  been  induced  in  a  certain  direction — say, 
so  as  to  send  a  current  from  brush  1  to  brush  2. 
At  the  moment  that  the  position  of  Fig.  69  is 
reached  the  armature  is  short-circuited  by. each 
brush.  This  will  be  of  no  great  disadvantage 
because,  as  we  know,  at  this  position  no  E.M.F. 
is  induced.  As  the  rotation  of  the  commutator 
proceeds,  the  half  shown  black  in  the  diagrams 
will  come  in  contact  with  brush  2,  and  the  white 
half  will  touch  brush  1.  Now,  it  must  be 
remembered  that  the  electrical  pressure  will 
be  in  the  reversed  direction;  but,  at  the  same 
time,  the  connections  with  the  outer  circuit  have  been  changed,  so 


FIG.  69.  — Second 
Position  of  Com- 
mutator. 


72 


ELECTRICAL  ENGINEERING 


that  the  current  (as  shown  in  Fig.  70)  will  again  proceed  from  brush 

1  to  2.     No  change  in  direction  of  the 

induced    current    will    follow    until    the 

commutator   from   the  position   Fig.   69 

has   turned   through   half   a    revolution. 

Reversal  of  the  current,  and  the  change 

of    brushes    to    rectify    it,    then    takes 

place.     This  is  repeated  at  all  subsequent 

half  turns. 

The  kind  of  current  so   produced  is 
not  really  a   constant  current,   such  as 
can   be   obtained   from   a   galvanic    cell, 
but  it  rises  to  a  maximum  and  falls  to 
nothing  repeatedly.    The  current  is  rep- 
resented  by  the   curve   of   Fig.  71,   and 
consists  of  half  waves  all   directed  up- 
wards.    The  peak  of  each  wave  corresponds  to  an  armature  position 
as  shown  in  Fig.  64,  and  the  zero  positions  show  the  absence  of  E.M.F. 
at  a  quarter  turn  later. 

The  dynamo  we  have  described  is  generally  employed  for  the 
generation  of  very  small  currents.  As  mentioned  above,  we  can 
produce  in  the  small  armature  a  comparatively  great  E.M.F.,  by 
employing  very  many  fine  windings.  Naturally,  we  obtain  from 
this  armature  only  a  very  small  current  on  account  of  the  fine  wire 
used  on  the  armature. 

Tt  is  sometimes  very  useful  to  get  a  pressure  of  100  volts  from 


FIG.  70.— Third   Position  of 
Commutator. 


\A 


FIG.  71. — Rectified  Current. 


such  a  small  portable  machine.  If  a  galvanic  battery  were  used  100 
cells  would  be  required.  This  would  be  more  bulky  than  a  small 
dynamo,  and  have  the  further  inconvenience  of  requiring  recharging 
from  time  to  time. 

This  simple  form  of  a  dynamo  is  therefore  used  as  current  gener- 
.ator  for  certain  tests,  such  as  that  of  insulation.  If,  we  require,  for 
instance,  to  examine  if  a  line  is  well  insulated  from  earth,  we  should 


ELECTRICAL  MACHINES 


73 


connect  one  terminal  of  the  dynamo  with  earth,  and  from  the  sec- 
ond terminal  lead  a  wire 
to  a  galvanometer — an  in- 
strument similar  to  an 
ammeter.  From  the  sec- 
ond galvanometer  termi- 
nal a  wire  is  led  to  the  line 
to  be  tested.  Then  the 
armature  is  turned  quick- 
ly. If  the  line  is  well  in- 
sulated from  earth,  then 
although  the  machine 
produces  an  E.M.F.,  no 
current  results,  and  the 
pointer  of  the  galvanom- 
eter remains  in  its  posi- 
tion of  rest.  If,  on  the 
other  hand,  the  insulation 
is  defective,  the  E.M.F.  of 
100  volts,  produced  in  the 
armature,  will  be  able  to 

send  a  current  through  the  circuit,  and  the  pointer  of  the  galvanome- 
ter will  be  deflected. 

We  can,  further,  note  from  the  force  which  has  to  be  exerted  for 
turning  the  armature  whether  the  machine  is  supplying  any  current 
or  not.  In  the  former  case  it  is  rather  difficult  to  turn  the  armature, 
because  electric  power  is  produced  in  the  machine.  In  the  latter 
case  the  turning  is  much  easier,  for,  as  there  is  no  current,  no  electric 
power  can  exist,  although  we  have  E.M.F.  present.  Thus,  only  such 
power  must  be  exerted  as  may  be  required  to  overcome  the  friction 
of  the  armature  and  the  gearing. 

This  machine,  which  is  generally  called  a  magneto,  is  also  used 
for  ringing  the  kind  of  electric  bells  that  are  often  used  in  connection 
with  telephones.  A  complete  machine  is  shown  in  Fig.  72. 


FIG.    72. — Magneto-Electric   Machine 
(Berliner  Telephone  Manufacturers'  Co.). 


CHAPTER  m 

THE  CONTINUOUS  CURRENT  DYNAMO 

The  Ring  Armature 

WITH  large  dynamos,  such  as  are  employed  for  electric  lighting  or 
power  transmission,  the  Siemens  armature  is  not  used.  In  these 
cases  the  ring  armature  invented  by  Gramme  is  sometimes  employed, 
particularly  for  arc  lighting. 


4- 

FIG.  73. — Ring  Armature. 

This  will  now  be  described.  It  is  shown  in  Fig.  73,  and  it  will  be 
seen  that  it  consists  of  a  ring-shaped  iron  core,  which  is  not  cast  or 
forged  in  one  piece,  but  built  up  from  a  great  number  of  thin  sheets 

74 


THE  CONTINUOUS  CURRENT  DYNAMO 


75 


of  soft  iron.  Over  the  ring  an  insulated  wire  is  wound  in  many  turns. 
The  ends  of  the  windings  are  soldered  together,  so  that  the  whole 
armature  winding  forms  a  circuit  closed  on  itself.  This  is  called  a 
closed-coil  armature  in  opposition  to  the  open-coil  type,  which  is, 
for  instance,  represented  by  the  simple  Siemens  armature.  If,  now, 
the  armature  rotates  between  the  poles  N  and  S,  an  E.M.F.  will  be 
produced  in  each  wire.  We  have  to  examine  how  the  different 
E.M.F.'s  produced  in  the  wires  behave  towards  each  other. 

We  must,  first  of  all,  be  clear  about  the  course  of  the  lines  of 
force.  To  the  latter,  coming  from  the  north,  and  going  to  the  south 
pole  a  way  is  offered  through  the  armature.  They  can  either  make 
a  bend,  and  go  through  both  halves  of  the  iron  core,  or  they  can 


FIG.  74. — Lines  of  Force  through  Ring  Armature. 


go  the  shortest  way,  directly  through  the  interior  of  the  iron  core,  to 
the  south  pole.  The  first  way  offers  much  the  lower  resistance, 
because  the  path  is  only  through  iron  having  a  small  magnetic 
resistance.  Hence,  most  of  the  lines  of  force  will  pass  round  the  iron, 
and  a  small  number  only  will  go  through  the  inside  of  the  ring  (see 
Fig.  74). 

We  have  next  to  carefully  distinguish  between  the  outer  and  the 
inner  wires  of  the  armature  winding.  The  outer  wires  cross  the  total 
number  of  lines  of  force  in  the  air  gap  in  passing  the  north  or  south 
pole.  The  result  is  that  in  the  outer  wires  a  considerable  E.M.F.  is 
induced,  the  direction  of  which  we  can  determine  if  the  direction  of 
rotation  is  given.  In  all  the  outer  wires  passing  the  north  pole  an 
E.M.F.  in  one  direction,  in  the  wires  passing  the  south  pole  an 


76  ELECTRICAL   ENGINEERING 

E.M.F.  in  the  opposite  direction,  is  induced.  From  the  inner  wires 
and  the  lateral  parts  of  the  windings  very  little  E.M.F.  is  obtained, 
because  very  few  lines  of  force  cross  them.  Hence  the  really  effec- 
tive portion  of  each  winding  is  the  outer  wire,  the  other  parts  of  the 
winding  serving  for  connecting  each  wire  with  the  next  one.  By 
means  of  these  connections  the  pressures  produced  by  the  outer  wires 
within  the  embrace  of  the  poles  are  placed  in  series.  The  wires  in 
the  space  between  the  upper  tips  and  the  lower  tips  of  the  poles, 
called  the  neutral  zone,  are  ineffective,  and  serve  as  connecting  wires 
only. 

When  this  ring  armature  rotates  no  current  will  circulate  through 
its  wires,  for  the  E.M.F.  produced  by  one  pole  is  equal  and  opposite 
to  that  produced  by  the  other  pole.  Hence  we  have  exactly  the  same 
case  as  in  Fig.  33,  where  we  had  two  cells  in  parallel  without  any- 
external  connection,  so  that  the  cells  were  in  opposition.  Immedi- 
ately we  provide  an  outer  path  the  two  pressures  combine,  and  send 
a  current  in  the  same  direction  to  feed  the  outside  circuit,  as  shown 
in  Fig.  34. 

With  the  ring  armature  we  can  get  connection  with  an  outer 
circuit  by  removing  the  insulation  from  the  portions  of  the  wires 
lying  on  the  outside  surface  of  the  ring,  and  fixing  brushes  in  the 
neutral  zone,  which  rub  on  the  bared  wire.  On  joining  the  + 
and  —  brushes  (see  Fig.  73)  to  lamps,  etc.,  the  right-  and  left-hand 
windings  now  work  in  conjunction,  and  each  supplies  half  the  current 
passing  out  from  the  brushes. 

It  is  not  very  usual  to  collect  the  E.M.F.  by  baring  the  external 
wires.  It  is  much  more  usual  to  have  a  special  part,  the  commutator. 


FIG.  75. — Ring  Armature  (British  Schuckert  Co.). 

For  this  purpose  every  turn  (or  every  second,  third,  or  other  turn, 
according  to  circumstances)  is  connected  by  a  soldered  wire  with  a 
bar  or  segment  of  hard  copper.  The  single  segments  are  of  wedge 


THE  CONTINUOUS  CURRENT  DYNAMO 


77 


shape,  and  are  insulated  from  each  other  by  means  of  thin  sheet-mica. 
The  segments  are  then  fixed  on  a  metal  cylinder,  from  which  they  are 
insulated.  Along  the  surface  of  the  segments  brushes  are  so  fixed 
that  between  them  and  the  segments  there  is  not  much  friction. 
The  effect  is  just  the  same  as  if  the  wires  were  made  to  slide 
on  the  brushes.  Fig.  75  is  an  illustration  of  a  ring  armature 


ZZL 


FIG.  76. — Section  of  Ring  Armature  and  Commutator. 


with   commutator,   and  Fig.  76   gives   a   cross-section  of  such  an 
armature. 

We  have  still  to  explain  why  the  armature  is  not  made  from 
solid  iron,  but  is  built  up  from  a  number  of  thin  iron  discs,  and 
provided  with  insulation,  so  as  to 
separate  the  iron  discs  from  each 
other.  If  we  let  a  solid  iron  ring 
rotate  very  quickly  in  a  magnetic  field, 
it  will,  after  a  short  time,  become 
exceedingly  hot.  This  is  explained  by 
the  fact  that  the  ring  crosses  magnetic 
lines  of  force,  hence  inducing  electro- 
motive forces,  which  produce  currents. 

Let  us  now  consider  what  would 
happen,  during  the  rotation  in  a  mag- 
netic field,  in  a  solid  piece  of  iron 
cut  from  the  armature  along  its  axis.    This  piece  would  have  a 
shape  similar  to  that  of  a  commutator  segment  (see  Fig.  77). 

This  armature  sector  would  not  cross  the  lines  of  force  symmetri- 
cally. The  uppermost  part  crosses  very  many,  the  lowermost  part 
hardly  any,  and  the  intermediate  parts  always  a  less  and  less  number 
of  lines  of  force,  according  to  their  distance  from  the  surface.  If 


FIG.  77. — Eddy  Currents  in  Iron. 


78  ELECTRICAL  ENGINEERING 

we  now  imagine  the  sector  to  be  divided  into  several  strips  (see 
Fig.  78),   these  strips  will  represent  electric  conductors,  which,  on 
rotation,  cross  lines  of  force,  so  that  an  E.M.F. 
is  induced  in  them,  which  will  be  greatest  in 
the  uppermost  strip,  smaller  in  the  second, 
finally  smallest  in  the  innermost  strip.     If  we 
consider,  for  instance,  the  uppermost  and  the 
78-~ Eddy  Currents    iowermOst  strips,  then,  if  the  E.M.F.  induced 
in  the  former  be,  say,  1  volt,  that  induced  in 
the  latter  might  be  -J  volt  only.     As  all  these 

conductors  form  together  one  piece,  they  are  connected  electrically 
with  each  other.  Against  the  large  E.M.F.  of  the  uppermost  strip 
a  small  E.M.F.  only  of  the  lowermost  strip  will  act.  Thus  in  the 
closed  circuit  a  current  will  flow  which  is  produced  by  the  difference 
of  the  two  electro-motive  forces.  This  difference  is  f  volt  in  our 
example.  This  E.M.F.  can  produce  in  a  solid  iron  bar,  which  has  a 
very  low  electrical  resistance,  exceedingly  strong  currents.  These  are 
transformed  into  heat,  and  may  make  a  solid  iron  bar  red  hot  after 
a  short  time. 

Hence  the  generation  of  strong  eddy  currents  is  prevented  by 
building  up  the  iron  core  of  thin  sheet-iron  discs  and  very  thin 
paper  alternately  (marked,  in  Fig.  76,  by  vertically  hatching).  The 
single  discs  are  insulated  from  each  other  by  paper  layers  or  insulation 
painted  upon  the  sheets  of  iron  themselves.  An  E.M.F.  is  now, 
of  course,  induced  in  each  of  the  iron  sheets.  But  if  we,  for  instance, 
assume  that  the  number  of  discs  required  for  building  up  the  arma- 
ture be  200,  then  the  E.M.F.  produced  in  the  uppermost  part  of 
each  of  the  discs  will  be  only  the  200th  part  of  that  produced  in 
the  full  armature  length,  i.e.  3^  volt.  Further,  this  far  smaller  E.M.F. 
has  to  flow  along  a  way,  which  offers  to  it  a  very  high  resistance.  To 
come  to  the  innermost  part  of  the  armature  disc,  the  current  has  to 
flow  through  the  very  thin  iron.  As  the  resistance  of  the  latter  is 
a  very  considerable  one,  the  eddy  currents  will  be  far  smaller  than 
those  occurring  with  the  solid  iron  armature.  As  a  matter  of  fact, 
the  temperature  rise  of  a  well-designed  armature  over  that  of  sur- 
rounding air  does  not  exceed  70°  to  90°  Fahr.  This  heating,  however, 
is  to  a  considerable  extent  produced  by  the  current  in  the  armature 
conductors;  the  eddy  currents  themselves  alone  cause  a  much  less 
rise  of  temperature. 

The  method  of  building  up  the  armature  out  of  single  sheets  of 
iron  is  now  generally  followed.  A  slight  difference  in  the  construction 
happens,  inasmuch  as,  in  some  cases,  the  single  discs  are  insulated, 
not  by  thin  paper,  but  by  a  coat  of  varnish  or  other  special  com- 
pounds. The  ring  armature  is  particularly  suited  for  high  potentials, 
.since  wires  having  much  difference  of  potential  are  not  brought  near 
together.  Thus  such  windings  are  used  for  arc  dynamos,  when  the 
voltage  at  the  brushes  may  be  as  high  as  6000  volts.  It  is  well  to 
have  the  number  of  coils  a  multiple  of  the  poles,  so  that  the  E.M.F. 
between  brushes  will  balance  on  both  sides. 


THE  CONTINUOUS  CURRENT  DYNAMO 


79 


Drum  Armature 

The  interior  part  of  each  winding  of  a  ring  armature  is  useless 
for  the  generation  of  E.M.F.,  but  is  necessary  for  connecting  every 
conductor  with  the  next  one  so  that  the  E.M.F.'s  do  not  act  against 
each  other,  but  in  the  same  direction.  This  series-connection  of 
the  electro-motive  forces  may  be  obtained  in  another  way,  viz.  by 
connecting  opposite  conductors  through  a  wire  which  is  laid  over 
the  surface  of  the  armature,  as  with  the  Siemens  armature.  If  we 
consider  Fig.  79,  we  shall  see  that  4  of  the  12  armature  wires  are 


11  10 

FIG.  79. — Drum  Armature  Connections. 


situated  within  the  reach  of  the  north  pole  (viz.  Nos.  12,  1,  2,  and  3), 
4  other  wires  are  situated  within  the  reach  of  the  south  pole  (6,  7,  8, 
and  9),  and  the  remaining  4  wires  in  the  neutral  zone.  Thus  if  in 
the  wires  under  the  north  pole  an  E.M.F.  is  produced  which  may 
be  directed  from  the  spectator,  in  the  wires  under  the  south  pole 
an  E.M.F  is  produced  which  is  directed  towards  the  spectator; 
whereas  in  the  wires  4,  5,  11,  and  10  no  E.M.F.  at  all  is  produced. 
It  is  now  clear  that  we  get  a  proper  series-arrangement  of  the 
electro-motive  forces  if  we  connect  the  front  end  of  wire  1  with  the 
front  end  of  any  of  the  wires  6,  7,  8,  or  9.  It  would  be  the  nearest 
to  connect  wire  1  with  the  exactly  opposite  wire,  7.  But  this  would 
not  give  a  proper,  continuous  armature  winding;  for,  if  we  connect 


80  ELECTRICAL   ENGINEERING 

the  back  end  of  wire  7  with  the  opposite  wire,  we  come  back  again 
to  wire  1.  To  get  a  continuous  armature  winding,  we  have,  thus 
to  select  as  the  pitch  a  number  which  is  not  exactly  equal  to  half 
the  number  of  the  wires.  To  come  from  wire  1  to  wire  7,  we  have 
to  make  six  steps:  the  pitch  is,  therefore,  said  to  be  6.  Let  us  now 
select  as  the  pitch  the  next  smaller  number,  viz.  5.  We  have  then 
to  connect  on  the  front  wire  1  with  6.  On  the  back  of  the  armature 
we  have  to  connect  6  with  11.  The  connections  at  the  front  of  the 
armature  are  indicated  in  Fig.  79  by  full,  twice  bent  lines;  those  at 
the  back  of  the  armature,  by  dotted,  straight  lines.  In  proceeding 
with  the  connections,  we  come  to  a  front  connection  from  11  to  4, 
then  at  the  back  from  4  to  9,  front  from  9  to  2,  back  2  to  7,  front 

7  to  12,  back  12  to  5,  front  5  to  10,  back  10  to  3,  front  3  to  8,  back 

8  to  1 — that  is,  back  to  the  point  of  departure.     The  closed  circuit, 
which  is  formed  in  such  a  way,  comprises,  thus,  all  the  wires  of  the 
armature.     In  the  middle  of  each  of  the  front  connections  a  joint  is 
made  with  one  commutator-bar. 

This  armature  acts  exactly  like  a  ring-armature.  Let  us  assume 
the  brushes  to  lie  on  the  commutator-bars,  which  correspond  to  the 
wires  4  and  11,  and  5  and  10  respectively,  and  let  us  then  follow  the 
course  of  the  current.  There  are  two  ways  going  from  the  left-hand 
brush — one  to  wire  4,  the  other  one  to  wire  11.  On  the  first  way  we 
come  from  4  through  the  back  connection  to  the  wire  9,  in  which 
an  E.M.F.  directed  towards  the  spectator  is  induced  (indicated  by  a 
dot).  If  We  proceed  in  this  direction,  we  come  through  the  front 
connection  to  wire  2,  in  which  an  E.M.F. ,  directed  from  the  spectator 
(indicated  by  a  cross),  is  induced.  Thus  this  E.M.F.  wis  acting  in 
a  like  direction  to  the  first  one.  We  reach  now,  through  a  back 
connection,  wire  7,  through  a  front  connection  wire  12,  whereby  all 
the  electro-motive  forces  add  themselves,  and  come  further  through  a 
back  connection  to  5.  Wire  5  is  a  neutral  wire,  in  which  no  E.M.F. 
is  induced,  and  which  is  in  direct  connection  with  the  second  brush. 
The  current  can  thus  flow  from  the  second  brush  into  the  outer 
circuit.  This  brush  is,  therefore;  the  positive  one,  whereas  the  left 
brush  is  the  negative  one. 

If  we  follow  the  second  way,  which  is  offered  to  "the  current  from 
the  left  brush,  we  come  from  11  to  6,  from  6  to  1,  from  1  to  8,  from 
8  to  3,  in  which  wires  the  induced  electro-motive  forces  add  them- 
selves, and  at  last  from  3  to  10.  On  this  second  way  we  have  as 
many  series-connected  wires  as  in  the  first  way,  viz.  4  effective  and 
2  neutral  conductors.  The  E.M.F.  of  the  second  half  of  the  armature 
is  thus  equal  to  that  of  the  first  one.  Both  halves  of  the  armature  are 
connected  in  parallel,  like  the  halves  of  the  ring  armature.  Fig.  80 
shows,  further,  a  diagram  of  connections  for  an  armature  with  24 
conductors.  In  this  case  the  pitch  is  11,  and  it  can  be  seen  that 
the  result  is  the  same  as  with  the  armature  with  12  conductors. 


THE  CONTINUOUS  CURRENT  DYNAMO 


81 


To  get  one  continuous  drum-winding,   it  is  necessary  to   select 
as   pitch   an   odd  number.     If   in   the   last   example   we  made  the 


20     Jg— £     17 

FIG.   80. — Drum  Armature  Connections. 

step  =  10,  then  we  go  from  1  to  11,  from  11  to  21,  and  so  on,  and 
can  never  arrive  at  conductors  with  even  numbers,  but  combine  half 
of  the  conductors,  viz.  the  odd  ones  only,  in  a  closed  winding. 
Instead  of  one  continuous  armature  winding,  we  get  in  this  way  two 
entirely  separated  windings. 

On  the  other  hand,  it  is  not  necessary  to  make  the  pitch  just  equal 
.  to  a  number  smaller  by  one  than  the  half  of  all  wires.  With  24  wires 
we  could  make  the  pitch  either  11  or  13. 

The  pitch  on  the  front  need  not  always  be  equal  to  that  of  the 
back.  If  we  had  22  wires,  for  instance  (see  diagram,  Fig.  82),  we 
could  make  the  front  step  =  11,  the  back  step  =  9,  and  would  then  get 
a  winding  of  quite  the  same  kind  as  considered  before. 

It  should  be  noted  that  in  Figs.  79  and  80  the  number  of  coils  are 
even  (i.e.  one-half  number  of  conductors),  and  hence  a  wire  on  one 
side  of  the  armature*  is  not  connected  by  the  end  connections  to  a 
wire  diametrically  opposite.  Thus,  in  Fig.  80,  wire  No.  1  cannot  be 
connected  to  wire  No.  13  diametrically  opposite,  so  that  they  do  not 
commutate  under  the  brushes  simultaneously.  In  the  winding 
shown  in  Fig.  82,  the  number  of  coils  is  odd,  so  that,  if  desired,  wires 
diametrically  opposite  could  be  connected  by  the  end  connecters. 


82 


ELECTRICAL  ENGINEERING 


The  method  more  often  used  in  practice  is  not  as  shown  in  Figs.  79, 
80,  82,  but  as  shown  in  Fig.  81,  where  the  coils  are  in  two  layers. 
As  shown  in  the  figure,  the  outer  layer  is  in  multiple  with  the  lower, 
and  due  to  this  there  may  be  a  little  unbalancing.  To  make  this 
perfect,  in  winding,  instead  of  proceeding  around  in  one  layer,  the 


connections  are  made  to  wires  first  in  the  lower  layer  and  then  in 
the  upper. 

The  drum  armature  was  first  employed  by  Hefner-Alteneck.  It 
has  certain  advantages  over  the  ring  armature.  Since  no  wires  go 
through  the  interior  of  the  armature,  for  small  armatures  the  iron 
discs  may  be  fixed  directly  on  the  armature  shaft.  The  construction 
of  the  armature  becomes  therefore  cheaper,  and  the  winding  simpler. 
Also  for  larger  armatures  the  drum  winding  is  preferred  to  the  ring 
winding. 

Special  means  must  be  provided  to  prevent  the  conductors  from 
sliding  on  the  smooth  armature  surface. '  For  this  purpose  on  the 
surface  of  the  armature  sometimes  grooves  are  made,  and  rods  are 


THE   CONTINUOUS   CURRENT  DYNAMO  83 

put  into  these  grooves,  which  prevent  the  conductors  from  sliding. 


FIG.   82. — Drum  Armature  Connections. 


VtfUUl/1/7 


These  rods  are  called  driving-keys.  For  smaller  armatures  about  4 
to  10  driving-keys  are  required. 

Generally    this    difficulty    is    overcome    in    quite   another    way, 
viz.   by   placing   the   conductors    themselves    into   slots,   which  are 

either  cut  in  the  complete 
armature  core,  or  stamped 
in  the  single  discs  which  are 
fixed  on  the  shaft,  so  that 
\  f  \  f  ^ey  form  continuous  slots, 

V  /  \ I         into     which     the     armature 

U J  «• ^         wires  are  laid.     In  this  case 

FIG.  83. — Open  FIG.  84. — Nearly  '  there  are  many  slots,  which 

Slots.  Closed  Slots.         are  Very   near  one  another. 

Between  the  single  slots  the 

teeth  or  small  bridges  of  the  armature  iron  project  In  Figs.  83  and 
84  portions  of  toothed  discs  are  shown.  Fig.  85  shows  a  toothed 
drum  armature  without,  and  Fig.  86  one  with  its  winding,  which 
lies  well  protected  in  the  slots.  These  armatures  are  called  toothed 
armatures  in  opposition  to  the  above  described  smooth  armatures, 
although  the  latter  may  have  some  slots  for  the  keys. 

Both  the  ring  and  the  drum;  the  smooth  and  the  toothed  armatures 


ELECTRICAL  ENGINEERING 


are  capable  of  producing  a  continuous  current  of  nearly  the  same  kind 

as  that  delivered  by  a  battery.     As  a  rule,  the  armature  winding 

consists  of   a   great   number  of 

conductors;  and,  by  the  action 

of    the    commutator,   at    every 

moment   all   wires    within    the 

range    of  the  lines  of  force  act  |p  ®£^  V\ 

in  the  same  manner,   and  give 

E.M.F.;   whereas  with  the  Sie- 


FIG.  85. — Armature-core,  without 
Winding. 


FIG.  86. — Armatures,  Partially  Wound. 


mens    armature  the  pressure  oscillated   at  each  half  turn  between 
zero  and  its  maximum  value.     Certain  fluctuations  do,  however,  take 


FIG.  87. — Finished  Armatures, 
place  also  with  these  armatures.     It  might  happen  that  there  are  at 


FIG.  88. — Wound  Drum  Armature, 
one  time  20,  whilst  in  the  next  moment  there  are  21  slots  under  the 


THE   CONTINUOUS   CURRENT  DYNAMO 


85 


pole-shoe,  so  that  the  E.M.F.  induced  is  alternately  a  little  smaller 
and  a  little  larger,  but  generally  these  fluctuations  are  unimportant. 

The  bipolar  is  manufactured  in  America  for  small  dynamos  and 
motors  up  to  about  10  kilowatts,  and  in  a  few  installations  made  by 
the  old  Edison  Company  they  are  in  use  up  to  150  kilowatts.  Also 


FIG.  89.  — N.  Y.  C.  Locomotive  Motor  Armature. 

a  recent  type  of  locomotive  motor,  made  by  the  General  Electric 
Company,  has  been  adopted  by  the  New  York  Central  Railroad 
Company,  each  armature  being  capable  of  delivering  600  H.P.  Fig. 
89  shows  the  general  arrangement,  the  truck  frame  serving  as 
part  of  the  magnetic  circuit.  A  bipolar  winding  has  the  full  potential 
between  layers,  so  that  special  care  must  be  used  in  insulating,  par- 
ticularly on  the  ends. 


Magnet  System 

Permanent  steel  magnets  cannot  be  magnetized  to  as  high  a  degree 
as  eleotro-magnets.  Hence,  for  dynamos,  electro-magnets  are  exclu- 
sively employed.  Fig.  90  shows  a  two-pole  dynamo  with  horseshoe- 
shaped  magnets.  Over  the  arms  of  the  latter  two  coils  are  wound, 
so  as  to  drive  all  lines  of  force  in  the  same  direction  through  the  mag- 
net, and  to  make  one  pole  north,  the  other  one  south,  magnetic. 

To  get  a  strong  magnetic  field  it  is,  as  we  know,  essential  to  have 
the  lines  of  force  going  as  far  as  possible  through  iron  only,  and  to 
make  the  way  through  air  or  any  other  non-magnetic  materials  as 
short  as  possible.  Hence  the  air  gap  between  armature  and  pole- 
shoes  is  kept  very  small.  It  is  obvious  that  we  can  approach  the 
armature  core  nearer  to  the  pole-shoe  with  a  toothed  armature  than 
with  a  smooth  one,  for  with  the  latter  the  winding  is  arranged  over 


86 


ELECTRICAL  ENGINEERING 


the  iron  core.     This  is  a  further  reason    why,  nowadays,  toothed 
armatures  are  generally  employed  for  dynamos. 

To  get  a  current  from  the  dynamo,  it  is  necessary  to  "excite"  the 
magnet  system,  i.e. ,  to  send  a  current  through  the  coils,  by  which  a 
magnetic  flux  is  produced.  The  current  for  exciting  the  magnet 
coils  may  be  taken  from  any  current  generator,  as,  for  instance,  a 


FIG.  90. — Magnetic  Circuit  of  Bipolar  Dynamo. 


galvanic  battery.  We  shall  see,  later  on,  that  it  is  not  necessary  to 
use  an  external  current  generator  for  this  purpose.  But  as  this  case 
is  the  simplest  one,  and  easiest  to  be  understood,  we  shall,  first  of  all, 
take  this  as  a  basis  for  our  consideration. 

If  we  send  a  current  through  the  magnet  coils,  and  let  the 
armature  rotate,  the  dynamo  will  produce  a  definite  voltage.  The 
E.M.F.  induced  in  each  conductor  is  larger  the  stronger  the  magnetic 
field  and  the  greater  the  speed  of  rotation  of  the  armature.  Since 
the  coils  of  each  half  of  the  armature  are  connected  in  series,  the 
E.M.F.  of  the  whole  armature  will  also  increase  with  the  number  of 
armature  wires.  If  we  turn  a  certain  armature,  firstly  with  a  speed 
of  500,  and  then  with  1000  revolutions  per  minute,  and  leave  un- 
changed the  strength  of  the  magnet  current,  then  the  E.M.F.  of  the 
armature  will  be  twice  as  much  in  the  second  case  as  in  the  first 
one.  If  we  strengthen  the  magnetizing  current,  the  E.M.F.  of  the 
armature  will  also  rise,  but  not  quite  in  the  same  proportion.  For, 
as  we  know,  there  is  a  limit  to  the  magnetization  of  iron,  and  if 
we  approach  this  limit,  a  great  increase  of  the  magnetizing  ampere- 
turns  causes  only  a  small  strengthening  of  the  magnetic  field.  By 


THE   CONTINUOUS  CURRENT   DYNAMO 


87 


means  of  a  diagram  we  can  make  this  clear  as  follows: — Let  us  draw 
a  horizontal  line  (Fig.  91),  and  divide  it  into  parts,  each  of  say 
1  cm.  The  length  of  1  cm.  represents,  then,  about  1000  ampere- 
turns.  Thus  we  mark  the 
first  division  with  1000,  the 
second  one  with  2000,  the 
third  one  with  3000,  and  so 
on.  Now  let  us  draw  a 
vertical  line  from  each  of 
these  divisions.  The  length 
of  these  lines  we  make 
equal  to  that  E.M.F.  which 
is  produced  by  the  armature 
at  a  constant  speed,  if  the 
magnet  arms  be  excited 
with  1000  ampere-turns  in 
the  first,  with  2000  in  the 
second,  3000  in  the  third 
case,  and  so  on.  A  height 
of  1  cm.  of  the  vertical  line 
has  to  reprecent  about  20 
volts.  We  observe  that, 
with  2000  ampere-turns,  the 


120 


100 


80 


60- 


40 


20 


Ampere  [Turns 


1000          2000       3000        4000        5000 
FIG.    91. — Magnetization    Characteristic. 


voltage  is  nearly  double  of 
that  with  1000  ampere-turns. 
But  as  the  excitation  grows  to  3000,  the  increase  of  the  voltage  is 
slower.  At  still  higher  excitations  the  voltage,  increases  but  less  and 
less.  If  we  connect  all  the  ends  of  the  vertical  lines  by  a  line,  we 
get  a  bent  line  or  a  curve.  This  line  is  steep  at  its  beginning,  and 
becomes  rather  flat  at  its  end.  This  curve  is  called  the  electro-motive 
force  characteristic  of  a  dynamo  on  open  circuit.  A  simpler  name  is 
the  magnetization  characteristic  or  saturation  curve. 

To  obtain  from  the  dynamo  that  we  are  considering  a  voltage  of 
110,  we  want  on  the  magnet  limbs  about  4000  ampere-turns.  We 
can  get  this  by  sending  through  a  coil  of  thick  wire,  with  40  windings, 
a  current  of  100  amps.,  or  by  sending  through  a  coil  of  fine  wire,  with 
2000  windings,  2  amps.,  and  so  on.  As,  in  the  first  case,  the  few 
windings  of  thick  wire  have  a  small  resistance,  the  voltage  required 
for  this  coil  is  small,  say  about  two  volts ;  whereas,  in  the  latter  case, 
the  numerous  windings  of  the  fine  wire  have  a  very  high  resistance, 
and  therefore  a  much  larger  voltage — about  100  volts — is  required. 
The  output  in  watts,  however,  which  has  to  be  spent  for  excitation 
is  practically  the  same  in  all  cases.  In  our  examples,  for  instance, 
it  would  be  200  watts. 

If  the  dynamo  delivers  current  to  an  outer  circuit,  the  voltage  on 
the  brushes  decreases.  We  have  observed  the  same  case  with  the 


ELECTRICAL   ENGINEERING 


loo 


battery.  The  dynamo  armature  has  a  definite  internal  resistance. 
If  now  through  the  armature  a  current  is  flowing,  the  resistance 
consumes  a  certain  voltage.  Thus  the  terminal  voltage,  i.e.  the 
voltage  of  the  brushes,,  is  smaller  than  the  E.M.F.  induced  in  the 
armature.  The  larger  the  current  the  greater  will,  as  we  know,  be 
the  voltage  drop.  . 

But  there  is,  in  addition  to  the  internal  resistance,  a  further 
reason  which  causes  this  voltage  drop.  The  currents  flowing  in  the 
armature  exert  a  reaction  on  the  magnetic  field,  so  as  to  weaken  the 
latter.  We  shall  deal,  later  on,  separately  with  the  question  of  the 

armature    reaction.     For    the 

120  present  moment  it  is  sufficient 

to  know  that  the  armature 
reaction  has  an  effect  similar 
to  the  ohmic  resistance  of 
the  armature.  Both  cause  a 
voltage  drop  at  an  increased 
load. 

We  can  make  this  clearer 
by  means  of  a  diagram.  Let 
us  assume  that  the  magnet 
system  be  magnetized  con- 
stantly by  4000  ampere-turns, 
and  that  the  current  taken 
from  the  armature  be  10,  20, 
30  amps.,  etc.,  respectively. 
1  cm.  on  the  horizontal  line 
may  represent  10  amps,  (see 
Fig.  92).  On  the  vertical  we 
plot  the  voltages  as  before. 


4-0 


Amperes 


10 


30 


FIG.   92. — Closed    Circuit   Characteristic.     Tr  ,,       , 

It  tne  dynamo  does  not  supply 

any    current,    its    pressure    is 

about  110  volts.  If  it  supplies  a  current  of  10  amps.,  the  pressure 
would  go  down  to  109  volts,  at  a  current  of  20  amps,  to  107  volts,  at 
30  amps,  to  103  volts,  and  so  on.  By  connecting  all  the  ends  of 
the  vertical  lines  we  get  a  curve,  which  shows  the  decrease  of  the 
voltage  with  increasing  load.  This  curve  is  called  the  closed  circuit 
characteristic  or  load  characteristic. 

It  is.  of  course,  desirable  in  many  cases  to  get  a  constant  dynamo 
voltage  at  a  varying  load.  If,  for  instance,  a  dynamo  supplies 
current  for  a  lighting  plant,  it  would  be  very  objectionable,  if  the 
voltage  fell  from  110  to  103  volts,  as  we  switched  on  more  lamps. 
To  bring  the  voltage  back  to  its  normal  value  of  110  volts,  it  is 
therefore  necessary  to  increase  the  number  of  ampere-turns,  so  that, 
for  instance  instead  of  4000  ampere-turns,  4200,  4500,  5000,  and 
so  on,  ampere-turns  may  be  produced.  This  can  be  effected  by 


THE  CONTINUOUS  CURRENT  DYNAMO 


89 


means  of  a  regulating  resistance.  To  understand  the  action  of 
a  regulating  resistance,  let  us  consider  the  following  example. 
Assume  a  dynamo,  giving  a  voltage  of  110,  which  requires  for 
excitation  at  no  load  4000  ampere-turns.  The  magnet  coils  consist 
of  2000  windings,  having  a  resistance  of  50&>.  Hence,  if  we  connect 
these  magnet  coils  with  a  voltage  of  100,  the  current  would  be 
2  amps.,  and  the  number  of  ampere-turns  2000x2  =  4000,  i.e.  just 
what  we  want.  But  let  us  now  connect  the  magnet  coils  with 
110  volts;  then  the  magnet  current  would  be  2.2  amps.,  and  the 
number  of  ampere- turns  2.2x2000  =  4400.  As  we  have  previously 
learned,  an  additional  number  of  ampere-turns  increases  the  voltage 
so  that  in  our  case  the  pressure 
would  rise  to,  say,  about  117  volts. 
To  get  only  a  voltage  of  110,  we 
have  to  connect  a  resistance  of  5co 
in  series  with  the  coils.  Then  the 
current  becomes  again  2  amps.,  and 
the  number  of  ampere-turns  4000. 
But  if  we  now  load  the  dynamo,  its 
voltage  will  come  down  to,  say, 
about  109  volts.  By  increasing  the 
number  of  ampere-turns,  or,  what 
is  the  same,  by  increasing  the  mag- 
netizing current,  we  can  increase 
the  voltage.  Thus  we  have  to  do 
nothing  but  switch  out  a  part  of 
the  resistance  connected  in  series 
with  the  coils.  This  is  effected  by 
approaching  the  lever  of  the  adjust- 
able resistance  to  its  position  of 
short  circuit.  The  greater  the  load 
of  the  dynamo,  the  larger  has  to  be 
the  magnetizing  current,  the  more 
resistance  we  have  therefore  to  cut 
out  of  the  circuit. 

Fig.  93  serves  as  an  illustration 
for    a    regulating    resistance.     The 

connection  with  the  coil  and  the  circuit  is  made  as  follows: — The 
centre-point  of  the  lever  K  is  connected  with  one  pole  of  the 
battery,  and  the  last  contact  on  the  left  with  one  end  of  the  coil. 
The  other  end  of  the  coil  is  connected  directly  with  the  second 
battery  pole.  If  the  lever  of  the  regulating  resistance  is  over  the  last 
contact  on  the  left  marked  0,  the  current  can  go  from  the  battery  to 
the  magnet  coil  directly,  without  flowing  through  the  resistance  spirals. 
We  say,  then,  that  the  resistance  is  short-circuited.  The  further  we 


FIG.    93.  — Regulating    Resistance. 


90 


ELECTRICAL   ENGINEERING 


move  the  lever  to  the  right,  the  more  resistance  spirals  the  current 
has  to  flow  through. 


FIG.   94. — Enclosed  Regulating  Resistance  (Berend  &  Co.). 

Fig.  94  shows  a  regulating  resistance  covered  with  stamped  sheet 
iron. 

Figs.  95  and  96  show  two  rheostats  as  made  by  the  General 
Electric  Company,  the  first  having  a  resistance  of  20  ohms  and  a 
carrying  capacity  of  10  amperes,  and  the  second  a  resistance  of  832 
ohms  and  a  carrying  capacity  of  75  amperes.  The  formula  for  the 
E.M.F.  of  a  direct  current  dynamo  is  derived  as  follows:  Let  n=the 
number  of  coils  in  series  between  brushes  =  number  of  external  con- 
tacts divided  by  4  in  a  bipolar.  Let  <£  =  the  flux  going  from  pole 
to  pole  being  enclosed  by  the  coils  of  the  armature.  Let  N=the 
number  of  revolutions  of  armature  (in  a  bipolar)  per  second.  In 
connection  with  the  production  of  E.M.F.  in  a  dynamo,  it  has  been 
shown  and  illustrated  in  Fig.  65,  page  69,  that  in  each  revolution 
of  the  armature  coil,  starting  say  as  shown  in  Fig.  65,  the  coil  is 
filled  full  of  lines  of  force  twice  and  emptied  twice,  thus  four  times 


THE  CONTINUOUS  CURRENT  DYNAMO 


91 


in  each  revolution  does  the  coil  cut  all  the  lines  of  force  <j>.    Thus, 
since  a  volt  is  produced  by  cutting  100,000,000  lines   of  force  per 

second,  the  average  volts  produced  per  coil=  ^     ^. 

iUU,UUU;UUu 


But  there 


FIG.  95. 


FIG.  96. 


Field  Rheostats, 
are  n  coils  in  series  between  brushes.      Therefore  the  E.M.F.  of  a 


D.C.  dynamo  having  n  coils  on  its  armature  =  1QQ  QQQ  QQQ.  On  a  two- 
pole  dynamo  n=the  external  conductors  divided  by  4.  Representing 
the  external  conductors  by  C,  the  formula  becomes  ^. 


Self=excitation — Shunt  Dynamo 

As  mentioned  before,  it  is  not  necessary  to  use  current  from  an 
external  current  generator  for  exciting  a  dynamo.  It  would  be  the 
simplest  way  to  use  the  voltage  of  the  dynamo  armature,  which  was, 
for  instance,  110  volts  in  the  last  example,  for  feeding  the  magnet 
coils,  which  had  to  be  excited  with  100  volts  at  no  load,  and  with  a 
somewhat  higher  voltage  at  full  load  It  is  clear  that,  if  the  machine 
is  brought  to  its  full  voltage,  the  magnet  coils  may  be  connected 
without  any  further  difficulty  with  the  armature,  so  that  the  machine 


92 


ELECTRICAL  ENGINEERING 


3s  able  to  work  properly.  The  only  question  arising  is  now:  How  is 
it  possible  to  bring  the  machine  up  to  its  voltage  without  the  use  of 
an  external  current  generator  ? 

For  this  purpose,  that  property  of  the  iron  which  we  know  as 
residual  magnetism  is  of  great  advantage.  Iron  which  has  once  been 
magnetized  always  retains  some  traces  of  magnetism.  It  follows, 
therefore,  that  across  the  field  of  a  non-excited  dynamo  a  certain 
although  it  may  be  a  very  small,  number  of  lines  of  force  pass.  If  in 
this  very  weak  magnetic  field  an  armature  is  turned,  a  very  small 
E.M.F.  is  induced,  and  since  the  magnet  coils  are  connected  with  the 
armature,  the  small  E.M.F.  will  send  a  defined,  but  small  current 
through  the  magnet  coils.  We  hence  get  a  few  magnetizing  ampere- 
turns,  which  strengthen  the  residual  magnetism. 
The  strengthened  field  now  induces  a  larger  E.M.F., 
the  latter  again  causing  a  stronger  magnetizing 
current,  which  produces  a  stronger  magnetism,  so 
that,  in  this  way,  the  machine,  in  a  short  time  and 
without  an  outer  source  of  current,  is  brought  to  its 
full  voltage. 

It  is,  of  course,  necessary  to  connect  the  magnet 
coils  properly  with  the  armature,  that  is  to  say,  in 
such  a  way  that  'the  current  produced  by  the 
armature,  and  flowing  through  the  coil,  really 
strengthens  the  existent  magnetism.  If  we  con- 
nected the  magnet  coils  with  the  armature  in  a 
wrong  way,  then  the  current,  flowing  in  the  wrong 
direction,  would  not  strengthen  the  residual  mag- 
netism, but  destroy  it,  and  the  dynamo  would  give 
no  voltage. 

Werner  Siemens  was  the  first  to  find  this  prin- 
ciple of  self-excitation  of  dynamos. 

Fig.  97  is  a  diagram  of  connections  for  a  self- 
exciting  dynamo.  To  one  side  of  the  brushes  is 
connected  the  load  (the  small  circles  represent 
lamps,  connected  in  parallel),  on  the  other  side 
are  the  magnet  coils  (marked  by  a  zigzag  line). 
There  might  have  been  connected  also  a  resistance 
in  series  with  the  magnet  coils,  but  this  is  not  considered  in  the  dia- 
gram. The  two  circuits  which  branch  off  the  armature  terminals  are 
called,  respectively,  the  main  circuit  -and  the  shunt  circuit.  The 
magnet  coil  acts  as  a  shunt  to  the  main  circuit.  A  machine,  having 
connections  as  shown  in  Fig.  97,  is  called  a  shunt  dynamo. 

How  does  such  a   shunt  dynamo   behave  with  various   current 
intensities  in  the  main  circuit? 

Suppose,  first  of  all,  that  the  main  circuit  is  disconnected.     We 
now  have  the  circuit  closed  through  the  armature  and  the  coils,  and 


FIG.  97.— Shunt 
Dynamo  Con- 
nections. 


THE   CONTINUOUS   CURRENT   DYNAMO  93 

the  machine  therefore  comes  to  a  defined  voltage — say,  about  110 
volts.  Let  us  now  switch  on,  in  the  main  circuit,  a  number  of  lamps, 
so  that  the  dynamo  has  to  supply  a  current  of  10  amps.  As  a  con- 
sequence, the  terminal  voltage  of  the  dynamo  will  fall,  due  to  the 
ohmic  resistance  and  the  armature  reaction,  to,  say,  109  volts.  But, 
at  this  moment,  the  magnet  coils  are  now  no  longer  connected  with 
110,  but  only  with  109  volts.  The  magnetizing  current,  and  hence 
the  excitation,  will  therefore  become  smaller,  and  the  dynamo  voltage 
will  further  fall.  At  a  load  of  20  amps,  we  had,  with  the  separately 
excited  machine  (page  88),  a  voltage  of  107;  whereas  the  voltage  of 
a  self-excited  dynamo  at  the  same  load  will  be  only  about  105  volts, 
and  at  30  amps,  load — about  100  volts  (against  103  with  the  separately 
excited  dynamo).  Thus  the  self -excited  dynamo  shows  the  property 
of  the  separately  excited  machine — the  falling  of  the  voltage  with 
increasing  load — in  a  far  higher  degree.  Naturally,  we  may  use  here 
the  same  auxiliary  means — a  regulating  resistance — for  keeping  the 
voltage  constant,  as  before. 

The  magnet  coils  of  a  110-volt  dynamo  are,  for  instance,  wound 
generally  so  as  to  get,  at  no  load,  a  sufficient  excitation  for  producing 
in  the  armature  a  voltage  of  110,  even  if  the  voltage,  measured  at  the 
magnet  coil  terminals,  be  90  volts  only  The  remaining  20  volts  are 
then  absorbed  by  the  shunt  regulator.  At  an  increased  load,  the 
resistance  switched  into  the  shunt  circuit  has  to  be  diminished.  The 
more  the  load  increases,  the  more  resistance  of  the  shunt  regulator 
has  to  be  short-circuited; 

Fig.  98  shows  the  external  characteristic  of  a  shunt  dynamo  of 
modern  design.  The  line  OB  gives  the  current  values  delivered 


A 

9             10            1 

1             12 

1 

•»•*. 
2 

3          x 

4 

Tolts 

0 

R 

Amps. 
FIG.  98.— Shunt  Characteristic. 

by  the  dynamo,  and  the  line  OA  the  voltage  values.  When  one- 
quarter  load  is  put  on  at  5,  the  voltage  drops  at  1  below  the  normal 
value  shown  by  the  line  AC.  If  it  is  raised  again  by  means  of  the 


94 


ELECTRICAL  ENGINEERING 


field  rheostat  at  9,  and  another  one-quarter  load  is  put  on,  the  voltage 
will  again  fall,  a  little  more  this  time,  to  2,  and  so  on  up  to  full  load.  A 
well-designed  dynamo  will  drop  at  2  about  5  per  cent.  Some  machines 
will  entirely  lose  their  magnetism  when  one-quarter  load  is  put  on 
without  any  further  adjustment  of  the  field  rheostat,  particularly  in 
going  from  three-quarters  to  full  current,  the  voltage  dropping  and 


FIG.  99. — External  Characteristic. 

the  field  following  till  no  voltage  at  all  is  reached.  This  is  called 
unbuilding,  but  should  not  occur  under  ordinary  conditions  with 
well-designed  machines.  The  external  characteristic  without  adjust- 
ment of  field  rheostat  is  shown  in  Fig.  99.  In  this  case  the  voltage 
starting  at  a  drops  till  it  reaches  b,  when  the  machine  unbuilds  and 
the  voltage  goes  to  O. 

Series  Dynamo 


In  the  case  of  a  shunt  dynamo  the  magnet  coils  are  excited  with 
nearly  the  full  armature  voltage,  but  only  a  small  part  of  the 
armature  current  flows  through  them.  The  greatest  part  of  the 
current  goes  through  the  main  circuit.  There  is,  however,  possible 
another  method  of  connection  of  the  armature  with  the  magnet 
coils.  As  we  know,  we  can  get  any  desired  magnetic  effect  with  a  coil, 
having  but  few  windings,  through  which  a  strong  current  passes. 
Hence  we  may  send  the  whole  dynamo  current  through  a  coil  con- 
sisting of  a  few  windings  of  comparatively  thick  wire  (see  Fig.  100). 
In  this  case  the  connections  have  to  be  as  follows:  One  brush,  say 
the  right  one,  has  to  be  connected  with  the  main  circuit  directly. 
From  the  second  brush  no  connection  is  made  with  the  main  circuit, 
but  the  current  has  first  to  flow  through  the  magnet  coil,  and  then 
enters  the  main  circuit.  The  latter  is  again  shown  in  the  diagram 
as  consisting  of  lamps.  Thus  in  this  case  the  full  or  main  current 


THE  CONTINUOUS  CURRENT  DYNAMO 


95 


flows  through  the  magnet  windings.  This  is  called  a  series  wind- 
ing. 

As  the  armature,  the  magnet  coils  and  the  main  circuit  are 
connected  in  series,  this  dynamo  is  called  a  series  dynamo.  Let 
us  now  consider  the  behaviour  of  such  a  machine  with  various 
currents. 

If  we  disconnect  the  main  circuit,  then  the  whole  circuit  is  dis- 
connected, and  no  current  whatever  can  flow,  neither  in  the  mains 
nor  in  the  armature  nor  the  magnet  coils.  Notwithstanding,  a  very 
low  E.M.F.  is  produced  in  the  armature,  originating  in  the  residual 
magnetism.  No  strengthening  of  this  E.M.F.  can,  however,  take 
place.  Thus  the  voltage  of  the  machine  is,  with  the  main  circuit  open, 


FIG.  100.— Series  Dynamo  Connections. 

a  very  low  one,  or,  practically  speaking,  nothing  at  all.  If  we  now 
close  the  main  circuit,  a  current  produced  by  the  small  E.M.F.  flows 
through  the  magnet  coils.  As  a  consequence,  a  strengthening  of  the 
field  follows,  with  an  increase  of  the  E.M.F.,  producing  a  greater  cur- 
rent, and  causing  a  further  strengthening  of  the  field  as  before.  If 
the  main  resistance  happens  to  be  a  large  one,  the  current  (in  this  case 
the  main  current  as  well  as  the  magnetizing  current)  and  the  E.M.F. 
will  be  small  only.  But  if  we  diminish  the  main  resistance  (we  may 
do  so,  for  instance,  by  connecting  some  more  lamps  in  parallel  with 


96 


ELECTRICAL  ENGINEERING 


those  which  were  already  burning)  the  current  flowing  through  the 
whole  circuit  is  increased.  Thus  the  machine  will  be  more  strongly 
magnetized,  and  hence  produce  a  larger  E.M.F.  We  thus  learn  that 
the  E.M.F.  of  a  series  dynamo  will  grow  with  an  increasing  load,  quite 
opposite  to  the  case  of  the  shunt  dynamo. 

But  this  growing  of  the  E.M.F.  at  increasing  load  has  naturally 
an  end.  The  strength  of  the  magnetic  field  cannot  increase  continu- 
ally, but  remains  constant,  after  having  reached  a  certain  value. 
On  the  other  hand,  there  is  a  voltage  drop  in  the  armature,  which 
is  greater  the  larger  the  armature  current.  After  the  machine  has 
reached  a  certain  voltage,  it  therefore  follows  that,  since  the  strength 
of  the  magnetic  field  cannot  further  be  increased,  the  growing  voltage 
drop  in  the  armature  and  armature  reaction  must  cause  the  terminal 
voltage  to  fall  when  we  put  a  greater  load  on  the  dynamo. 

As  long  as  the  load  of  a  series  dynamo  is  not  too  high,  its 
terminal  voltage  grows  with  an  increasing  load:  if  now  the  load 

140T 


120 


100- - 


80 


60-- 


40-- 


20 


50 


100 


.150  200 

Amperes 


250 


FIG.  101.— Closed  Circuit  Characteristic  of  Series  Dynamo. 

be  further  increased,  the  voltage  remains  constant  for  a  certain 
period;  then  if  the  load  is  increased  once  again,  a  fall  of  pressure 
must  ensue. 

The  diagram,  Fig.  101,  shows  the  characteristic  curve  for  a  series 
dynamo. 


THE    CONTINUOUS  CURRENT  DYNAMO  97 

For  feeding  a  variable  number  of  glow  lamps  connected  in  parallel 
a  series  dynamo  cannot  be  employed,  for  the  voltage  of  the  dynamo 
would  vary  with  the  number  of  lamps  burning,  and  therefore  the 
lamps  would  from  time  to  time  vary  in  candle-power.  If  the  number 
of  lamps  burning  can  be  kept  constant,  lighting  with  a  series  dynamo 
is  possible,  although  this  is  seldom  done.  We  may  compare  the  dif- 
ference in  the  behaviour  of  the  shunt  and  series  dynamos  with  the 
difference  in  the  characters  of  two  men.  With  one,  his  effort  ceases 
if  he  has  to  do  more  work,  whereas  with  the  other,  a  greater  demand 
strengthens  his  resolve  and  his  power  up  to  a  certain  point,  beyond 
which,  if  his  task  be  increased,  he  also  breaks  down. 

The  series  dynamo  is  particularly  suited  for  series  arc  lighting, 
where  a  constant  current  is  desired  and  an  increased  voltage  when  the 
number  of  lamps  in  circuit  is  increased.  It  is  customary  to  work 
such  machines  at  the  point  D  (Fig.  101)  on  the  characteristic  curve, 
which  results  in  a  constant  current  with  varying  voltage.  Even  at 
short  circuit  on  such  a  machine,  the  current  does  not  increase  seriously 
(at  B,  Fig.  101).  The  large  armature  reaction,  to  be  explained  later, 
and  high  internal  resistance,  which  naturally  follow  from  a  large 
number  of  armature  turns,  and  high  voltage  generated,  hold  the 
increase  of  current  in  check.  Sometimes  series  generators  are  used 
as  boosters,  being  connected  to  constant  voltage  supplies  to  increase 
the  voltage  on  a  circuit  in  proportion  to  the  load,  tinder  such  con- 
ditions the  series  dynamo  would  be  operated  on  the  characteristic 
between  C  and  A,  the  voltage  under  such  conditions  rising  with  the 
load  which  is  desired.  In  the  latter  condition,  the  iron  of  the  mag- 
netic circuit  would  be  operated  below  saturation,  so  as  to  get  a  re- 
sponse to  the  increase  in  ampere  turns.  Also  the  dynamo  would 
be  designed  with  less  armature  reaction,  so  that  the  characteristic 
would  not  tend  to  drop,  as  shown  in  Fig.  101,  but  would  continue 
in  an  approximate  straight  line.  The  designer,  therefore,  lays  out 
his  machine  to  meet  the  conditions  imposed. 


Compound  Dynamo 

Generally,  it  is  required  from  a  dynamo  to  supply  constant  voltage 
at  varying  loads.  This  may  be  effected,  not  only  by  a  shunt  dynamo 
having  an  adjustable  shunt  resistance,  but  also  by  a  suitable  winding  of 
the  magnet  coils.  For  this  purpose  the  dynamo  is  provided,  besides 
the  shunt  winding,  with  a  series  winding,  which  latter  is  wound 
either  over  or  under  or  beside  the  former.  Care  must  be  taken  to 
wind  both  windings  in  such  a  way  that  they  may  act  in  the  same 
sense.  If  the  dynamo  does  not  supply  current,  the  series  winding 


98 


ELECTRICAL  ENGINEERING 


has  no  effect  at  all.  The  shunt  coil  only  has  to  be  considered, 
and  this  brings  the  machine  up  to  a  certain 
voltage — say  110,  for  instance.  If  the  machine 
supplies  current,  its  terminal  voltage  would 
fall,  if  there  were  only  a  shunt  coil;  but 
now,  since  the  main  current  flows  through  the 
series  coil,  the  number  of  ampere-turns  is  in- 
creased, and  the  magnetic  field  is  strengthened. 
With  a  proper  proportion  of  the  number  of 
the  two  windings,  we  can,  up  to  a  certain  limit, 
get  a  constant  or  nearly  constant  voltage  at 
varying  loads.  If,  however,  the  current  taken 
from  the  dynamo  exceeds  the  limit  allowed, 
then  the  voltage  falls.  The  diagram  of 
connections  for  such  a  machine  is  shown 
in  Fig.  102.  The  shunt  winding  is  indicated 
by  a  fine,  the  series  winding  by  a  thick,  zigzag 
line.  A  dynamo  whose  magnets  are  wound 
in  the  way  described  is  called  a  compound 
dynamo. 

With  a  series  coil,  having  a  sufficient 
number  of  turns,  we  may  also  obtain  the 
result  that,  at  an  increased  current,  the  voltage 
too  is  increased,  so  that  the  voltage  drop  in 
the  mains  which  grows  with  a  larger  current, 
is  compensated.  In  this  way  the  pressure  at  a 
place  distant  from  the  dynamo  may  be  kept 
constant.  The  dynamo  in  this  case  is  said  to  FIG.  102.— Compound 
be  over-compounded.  Dynamo  Connections. 


Types  of  Dynamos 


The  essential  parts  of  a  dynamo  are  the  magnetic  frame,  the 
armature,  and  the  commutator.  The  magnetic  frame  may  assume 
very  many  different  shapes.  For  the  actual  working  of  the  machine 
only  the  field  between  the  pole-shoes  is  of  special  importance. 
But,  to  get  a  strong  magnetic  action,  the  connection  between  the 
two  poles  must  be  made  of  iron.  The  shape  of  this  part  of  the 
magnetic  circuit  depends  on  the  choice  of  the  designer. 

The  magnetic  frame  may,  for  instance,  have  the  shape  of  a  horse- 
shoe, with  which  we  became  acquainted  in  the  previous  chapter. 
The  horseshoe  may  have  the  yoke  upwards,  so  that  the  magnet  stands 
in  a  manner  on  its  poles  (see  Fig.  103).  This  type,  which  has  been 


THE  CONTINUOUS  CURRENT  DYNAMO 


99 


employed  by  Edison,  and  is  therefore  called  the  Edison  type,  was  very 

common  some  time 
ago,  but  is  very  sel- 
dom built  now-a-days. 
When  horseshoe  mag- 
nets are  employed, 
they  are  generally 
built  with  the  yoke 
downwards,  so  that  the 
yoke  may  either  be 
used  as  base,  or  may 
be  cast  together  with 
the  base-plate.  This 
type  (see  Fig.  104)  is 
called  the  Kapp  type, 
after  Kapp,  who  first 
employed  it.  Fig.  105 
shows  a  machine  after 

the  Kapp  type,  but  the  construction  of  which  differs  somewhat  with 

regard  to  the  arrangements  of  the  bearings. 

It    is    not   necessary    to   employ    two    magnetizing  coils.      The 


FIG.  103.— Edison  Type.   FIG.  104. — Kapp  Type. 


FIG.  105. — Kapp  Type. 


magnetic  flux  may,  of  course,  be  obtained  as  well  by  one  coil,  having 
double  the  number  of  ampere-turns.  Fig.  106  shows  a  machine  which 
is  also  of  the  horseshoe  type,  but  with  the  windings  on  one  coil — on; 
that  placed  on  the  yoke.  In  this  case  the  pole-shoes  are  one  above 


100 


ELECTRICAL  ENGINEERING 


the  other,  whereas  they  are  arranged  side  by  side  in  all  the  types  we 
have  hitherto  considered. 

It  is  also  not  necessary  to  make  the  yoke  between  the  two  pole- 
shoes  in  one  piece,  but  it  may  be  divided  into  two  parts.     We  have 


FIG.  106.— C  Type. 


FIG.  107.— Manchester  Type. 


then  a  magnetic  circuit,  split  into  two  branches,  similarly  to  the 
branching  of  an  electric  circuit. 

Fig.  107  shows  a  scheme  of  such  a  dynamo.     The  upper  pole  is 
a  north,  the  lower  one  a  south  pole.     The  lines  of  force  go,  then, 


FIG.  108. — Manchester  Type. 


from  the  north  pole,  through  the  air  gap  and  the  armature  core, 
downwards  to  the  south  pole,  enter  the  yoke,  and  branch  to  the  right 
and  to  the  left.  They  go  upwards  through  the  two  vertical  columns 


THE  CONTINUOUS  CURRENT  DYNAMO 


101 


bearing  the  coils,  and  join  again  in  the  upper  yoke.    The  coils  have, 

of  course,  to  be  wound  so  that  both 
tend  to  generate  a  flux  of  lines  of 
force  directed  upwards.  Thus  the 
current  must,  seen  from  one  side, 
flow  in  the  same  direction  through 
the  coils.  The  machine  described 
here  is  called  a  Manchester  machine. 
Fig.  108  is  an  illustration  of  a  com- 
plete machine  of  this  type.  This 
•was  the  type  tf  magnetic  circuit  used 
in  the  old  Sprague  dynamo  of  several 
years  ago. 

The  magnet  coils  may  also  be 
arranged  in  another  way.  Fig.  109 
shows  a  type  of  machine  called  the 
Lahmeyer  or  semi-enclosed  type. 
With  this  machine  the  coils  are 
arranged  over  and  underneath  the 
pole-shoe,  and  the  yoke,  which 

PIG.  109. — Lahmeyer  type  (British   is   split   into    two   parts,   partially 
Schuckert  Co.\  encloses  the  machine.     Fig.  110  is 

also  built  after  this  type. 


FIG.  110.  — Lahmeyer  Type  (Mcschinenfabrik  Oerlikori). 


102 


ELECTRICAL  ENGINEERING 


The  Gramme  machine  (see  Fig.  Ill)  has  another  arrangement 
of  the  magnetic  field,  which  was  employed  in  the  early  days  of 
dynamo  building.  Its  magnetic  field  consists  of  a  double  magnetic 
circuit,  like  that  of  a  Manchester  or  Lahmeyer  machine.  With  the 
Gramme  machine,  however,  each  half  is  provided  with  two  coils,  one 
at  the  top  and  one  at  the  bottom. 

There  are,  besides  the  types  mentioned  hitherto,  a  great  number  of 
different  magnet  shapes,  but  which  are  very  seldom  used.  The  most 
usual  2-pole  machines  are  those  after  the  Lahmeyer  and  the  Kapp  type. 


FIG.  111. — Gramme  Dynamo. 

As  material  for  the  construction  of  field  magnets,  formerly  wrought 
iron  was  in  general  use,  because  this  can  be  magnetized  more 
strongly  than  other  kinds  of  iron.  Cast  iron  has  a  far  lower  magnetic 
conductivity,  not  much  more  than  half  that  of  wrought  iron.  Hence 
if  we  employ  cast  iron  we  have  to  make  the  cross-sectional  areas  of 
the  limbs  and  the  yoke  twice  as  big  as  with  wrought  iron,  in  order  to 
get  the  same  number  of  lines  of  force.  As,  however,  in  spite  of 
the  double  cross-sectional  areas,  the  cast-iron  machines  can  be  more 
cheaply  manufactured  than  the  wrought-iron  ones,  the  use  of  the 
latter  material  has  been  abandoned. 


THE  CONTINUOUS  CURRENT  DYNAMO  103 

Lately,  steel  makers  have  succeeded  in  manufacturing  by  a  process 
of  casting  a  material  similar  to  wrought  iron.  This  is  generally 
called  cast  steel.  It  is  of  quite  another  quality  to  the  hard  steel, 
such  as  is  used  for  tools  and  permanent  magnets.  The  magnetic 
properties  of  cast  steel  are  very  similar  to  those  of  wrought  iron. 
Hence,  in  making  the  magnet  frame  from  cast  steel,  we  can  make 
the  cross-sectional  areas  as  small  as  with  wrought  iron;  that  Is, 
equal  to  about  one-half  of  the  cross-sectional  areas  of  a  cast-iron 
frame.  On  the  other  hand,  cast  steel  is  more  expensive  than  cast 
iron.  Hence  none  of  these  materials  has  come  into  exclusive  use. 
In  some  cases  magnetic  frames  are  made  of  cast  steel,  in  other  cases 
cast  iron  is  employed  with  advantage. 


Output  of  a   Dynamo 

The  strength  of  current  which  can  be  taken  from  a  dynamo  is 
limited  chiefly  by  the  heating  of  the  armature  wires.  From  an 
armature  wound,  for  instance,  with  wire  of  No.  18  S.W.G.  we  cannot, 
of  course,  take  a  current  of  30  amps,  for  a  long  time.  As  the 
armature  has  two  parallel  circuits,  through  each  of  them  a  current 
of  15  amps,  would  flow,  which  would  obviously  heat  this  wire  far  too 
much. 

We  must,  however,  not  think  that  the  table  given  at  the  end  of 
the  first  chapter  is  a  standard  for  the  maximum  current  allowable  in 
armature  wires.  For,  firstly,  the  heating  allowable  for  armature 
conductors  is  a  far  higher  one  than  it  is  with  main  conductors;  and, 
secondly,  due  to  the  quick  rotation  of  the  armature,  the  conductors 
are  cooled  continuously  by  a  draught  of  air.  Thus  the  current 
density  used  for  armature  wires  varies  between  650  and  4500  amps, 
per  square  inch,  and  even  more  with  very  small  machines.  There  is 
no  general  rule  with  regard  to  current  densities  of  armature  wires, 
for  according  to  the  special  designs  of  armatures  the  draught  of  air 
produced  by  them  may  be  stronger  or  weaker.  With  regard  to 
the  heating  of  a  dynamo,  a  rise  of  the  dynamo-temperature  of  70° 
to  90°  Fahr.  over  that  of  the  surrounding  air  is  generally  considered 
allowable. 

Still,  with  any  given  type  of  dynamo,  the  maximum  current  allow- 
able is  determined  by  the  thickness  of  the  armature  wires.  Now,  the 
output  of  a  dynamo  is  determined  by  the  product  of  the  number  of 
amperes  by  the  number  of  volts.  Hence  we  have  to  examine  by 
what  factors  the  voltage  of  a  dynamo  is  determined. 

We  know  that  the  voltage  is  greater,  the  greater  the  number 
of  armature  wires,  the  stronger  the  magnetic  field  or  the  number 


104  ELECTRICAL  ENGINEERING 

of  lines  of  force  leaving  the  pole,  and  the  quicker  the  conductors  are 
moved.  Now,  the  number  of  armature  wires  cannot  be  indefinitely 
increased.  On  a  given  smooth  armature,  having  a  certain  distance 
from  the  pole-shoes,  a  limited  number  of  wires  only  can  be  fixed. 
Similarly,  into  the  slots  of  a  toothed  armature  a  certain  number  of 
wires  only  can  be  put.  If,  however,  we  employ  a  thicker  wire  in 
order  to  get  a  stronger  current  from  the  armature,  we  can,  on  the 
given  space  for  winding,  fix  a  smaller  number  of  wires  only;  that  is, 
with  given  armature  dimensions  we  can  wind  the  latter  either  for  a 
smaller  current  and  a  larger  voltage,  or  for  a  larger  current  and 
a  smaller  voltage.  Hence  we  may,  for  instance,  wind  an  armature 
so  as  to  get  from  it  either  10  amps,  at  a  voltage  of  110,  or  about  20 
amps,  at  a  voltage  of  55.  Thus  the  output  of  the  armature  remains 
about  the  same  in  both  cases,  provided  that  the  other  determinative 
factors  are  not  altered. 

These  determinative  factors  are  the  number  of  lines  of  force,  and 
the  number  of  revolutions  of  the  machine.  The  number  of  lines 
of  force  is  greater  the  larger  is  the  cross-sectional  area  of  the 
magnetic  frame,  and  the  more  it  is  saturated.  As  a  rule,  the  satura- 
tion is  never  pushed  to  its  limit,  as  in  this  case  an  extremely  large 
number  of  magnetizing  ampere-turns,  and  thus  very  big  magnet  coils, 
would  be  required  Generally  the  iron  is  magnetized  up  to  three- 
quarters  of  its  limit  of  saturation.  Hence,  if  a  field  of  a  threefold 
strength  is  required,  and  we  cannot  further  substantially  increase  the 
saturation,  it  will  be  essential  to  enlarge  the  cross-sectional  area  of 
the  magnet  limbs  about  threefold,  thus  making  the  machine  bigger 
and  heavier. 

Naturally  the  number  of  revolutions  of  a  machine  is  chosen  as 
great  as  possible.  With  smaller  dynamos,  up  to  an  output  of  about 
3000  watts,  a  speed  of  about  2000  revolutions  per  minute  is  generally 
employed.  If  a  machine  be  run  at  1000  revolutions  instead  of  2000, 
we  get  half  the  voltage,  and  hence  only  half  the  output. 

Another  factor  in  heating  an  armature  is  what  is  called  hysteresis. 
When  the  magnetism  in  the  iron  core  is  reversed,  which  occurs  once  in 
a  revolution  in  a  two-pole  machine,  the  molecules  of  iron  in  the  core 
tend  to  turn  about  with  the  magnetism.  This  naturally  cannot 
occur  only  very  partially.  The  effort  to  do  this  causes  rubbing  of 
the  molecules  one  upon  another  and  from  this  heating,  due  to  the 
friction  resulting.  Mr.  Charles  P.  Steinmetz  in  a  series  of  experiments 
\  showed  that  the  loss  in  hysteresis  expressed  in  watt-seconds  or  joules 

KB1'6 
per  cm3  and  cycle  of  magnetism^--— 7— ,  where  B  equals  flux  density 

per  centimeter,  and  K  is  a  constant  depending  upon  the  quality  of 
the  iron. 


THE  CONTINUOUS  CURRENT  DYNAMO  105 

This  loss  must  be  added  to  the  loss  in  the  copper  and  to  the  loss 
due  to  eddy  currents  in  copper  and  iron  (previously  discussed)  to 
give  the  total  armature  loss.  The  radiating  surface  must  then  be 
found,  from  which  the  total  loss  in  watts  per  square  inch  can  be 
determined.  Knowing  the  loss  per  square  inch,  the  temperature 
can  be  accurately  predicted,  for  the  rise  in  temperature  of  any  surface 
is  proportional  to  the  loss  of  energy  that  must  be  radiated  from  that 
surface.  Thus  from  the  surface  of  a  spool  a  watt  of  energy  from  each 
square  inch  will  raise  the  temperature  of  the  spool  about  70°  Cen- 
tigrade. A  loss  due  to  friction  and  I2R  of  brush  contact  on  a  com- 
mutator amounting  to  1  watt  per  square  inch  of  commutator  surface 
will  raise  the  temperature  of  the  commutator  10°. 


Multipolar    Dynamos 


Large  machines  cannot  be  built  without  considerable  difficulty  for 
very  high  speeds,  more  especially  if  they  have  to  be  coupled  directly 
to  steam  or  gas  engines.  In  the  latter  cases,  their  speed  must  not 
exceed  200  to  300  revolutions,  and  in  a  few  cases  only  it  may  come  to 
400  to  500  revolutions  per  minute.  With  dynamos  built  in  the  way 
we  have  already  described,  built  with  only  two  poles,  the  cross-sec- 
tional areas  of  the  magnetic  frame  would  have  to  be  made  very  large, 
and  the  whole  machine  would  become  too  bulky  and  expensive  when 
low  speeds  are  required. 

We  may,  however,  design  a  machine  from  another  point  of  view. 
Suppose  we  let  every  conductor  at  each  revolution  pass,  not  two 
only,  but  a  row  of  several  poles.  Then  we  get,  in  spite  of  the  lower 
speed,  as  large  a  number  of  alternations  as  with  a  high-speed  machine, 
and  the  poles  may  then  have  a  far  smaller  cross-sectional  area.  Fig. 
112shjws,for  instance,  a  4-pole  magnetic  frame.  The  magnet  coils 
are  wound  so  as  to  produce  north  and  south  po\es  alternately.  In 
our  example  the  upper  pole  would,  for  instance,  be  a  north  pole,  the 
one  to  the  right  a  south  pole,  the  lower  one  a  north,  and  the  one  to 
the  left  again  a  south  pole.  Hence  the  lines  of  force  leave  the  upper- 
most pole  and  enter  the  armature.  Half  of  the  lines  go  through 
the  armature  to  the  right,  enter  from  there  the  south  pole  to  the 
right,  and  come  back  again  through  the  upper  right  part  of  the 
yoke  to  the  upper  north  pole.  The  other  half  of  the  lines  pass  the 


106 


ELECTRICAL  ENGINEERING 


Fro.  112 — Four-Pole  Magnetic  Circuit. 


FIG.  133— Four-Pole  Dynamo  of  American  Manufacture 


THE   CONTINUOUS  CURRENT  DYNAMO 


107 


upper  left  quarter  of  the  armature  core,  enter  the  left  south 
pole,  and  then  come  back  through  the  left  upper  quarter  of  the 
yoke  to  the  upper  north  pole.  In  exactly  the  same  way,  we  can 
follow  the  course  of  the  lines  of  force  of  all  the  single  poles. 
The  yoke  may  be  circular,  or  of  polygon  shape.  Fig.  113  shows 
a  complete  4-pole  machine.  This  type  is  generally  used  for 
machines  having  an  output  of  about  10  to  50  kilowatts;  but  these 
limits  are  by  no  means  always  followed,  and  dynamos  for  a  far  smaller 
output  than  10  kilowatts,  and  sometimes  for  a  higher  output  than 
50  kilowatts,  are  made  with  four  poles.  But,  as  a  rule,  for  machines 


FIG.  114.— Six-Pole  Dynamo  (British  Schuckert  Co.). 


having  a  large  output,  6-,  8-,  and  more,  pole  magnetic  frames  are 
employed.  Fig.  114  shows  a  6-pole  dynamo  for  an  output  of 
about  100  kilowatts;  Fig.  115  the  18-pole  magnetic  frame  of  a 
dynamo,  designed  for  direct  coupling  to  a  slow-speed  steam-engine 
and  for  an  output  of  400  kilowatts. 

It  is  not  absolutely  necessary  that  with  multipolar  machines 
every  pole  be  provided  with  a  magnetizing  coil.  In  some  cases  there 
is  a  magnet  coil  over  every  second  pole  only.  Fig.  117  shows  a 
4-pole  dynamo,  having  only  two  poles  provided  with  magnet  coils. 
At  a  superficial  glance,  one  might  consider  it  to  be  a  2-pole  machine. 


108  ELECTRICAL   ENGINEERING 

The  essential  difference,  however,  between  such  a  machine  and  a 


FIG.  115.— Eighteen-Pole  Magnet  Frame  (Korting  Bros.}. 

2-pole  one  is,  that  with  the 
latter  the  opposite  poles  are 
different  ones,  for  instance,  a 
north  pole  on  the  right  and  a 
south  pole  on  the  left,  whereas, 
with  the  4-pole  machine  the 
opposite  poles  are  alike,  say, 
for  instance,  two  north  poles. 
If  we  follow  the  path  of  the 

lines   of   force    (see   Fig.  116), 
FIG.  116.— Two-Coil  Four-Pole  Dynamo.       we    find    that    they    ]eaye    the 

right  north  pole,  and,  after  having  passed  the  armature,  one-half 


THE  CONTINUOUS  CURRENT  DYNAMO         109 

of  them  enter  the  upper,  and  the  other  half  the  lower  of  the  poles, 
having  no  coils.  In  a  like  manner,  one-half  of  the  lines,  leaving 
the  left  north  pole,  enter  the  upper,  the  other  half  the  lower  one 
of  the  unwound  south  poles.  Thus  as  many  lines  enter  each  of 
the  unwound  poles  as  leave  each  of  the  wound  north  poles.  The 
strength  of  the  unwound  south  poles  is,  therefore,  as  great  as  that 
of  the  wound  north  poles.  The  former  are  called  consequent  poles. 
There  is  naturally  no  saving  of  wire  as  we  have  one  coil  only  for 


FIG.  11T. — Four-Pole  Dynamo  with  two  Coils. 

each  magnetic  circuit  against  two  with  the  usual  pole  arrangement, 
and  therefore  each  of  the  two  coils  has  to  have  twice  as  many 
ampere-turns  as  each  of  the  usual  four  coils. 

In  a  like  manner  we  could  wind  a  6-pole  machine  with  three 
coils,  and  an  8-pole  machine  with  four  coils.  This  construction 
is,  however,  very  seldom  employed. 

The  formula  for  the  E.M.F.  of  a  4-pole  dynamo  remains  the 
same  as  for  a  2-pole,  but  it  must  be  remembered  that  the  coils  in 
series  between  brushes  on  a  4-pole  multiple  winding  equal  the  external 
conductors  divided  by  8.  With  a  series  winding  (described  later) 
the  coils  in  series  may  equal  the  external  conductors  divided  by  4, 
as  with  a  2-pole  machine. 

Armatures  of  Multipolar  Dynamos 

The  armatures  of  multipolar  machines  may  be  wound  as  ring  or 
drum  armatures,  like  those  of  2-pole  machines.  The  ring  armature 
may  even  be  used  in  quite  an  unchanged  form  for  multipolar  machines. 


110 


ELECTRICAL  ENGINEERING 


The  machine  has  then  to  be  provided  with  as  many  brush- 
holder  studs  as  there  are  poles.  In  Fig.  118  a  4-pole  ring  armature 
is  shown.  If  we  imagine  it  rotating  clockwise,  then  in  the  outer  con- 
ductors, being  under  the  influence  of  the  north  pole,  currents  are 
induced  which  are  directed  from  the  spectator,  whereas,  in  the  wires, 
being  under  the  influence  of  the  south  pole,  currents  directed  towards 
the  spectator  are  induced,  which  are  marked  in  the  diagram  by  crosses 
and  dots  respectively.  As  we  see  now,  the  currents  induced  in  the 
upper  quarter  of  the  armature  are  directed  towards  the  left  upper 
brush,  thus  making  this  a  positive  one,  whereas  the  right  upper 


FIG.  118. — Four-Pole  Parallel  Ring  Armature. 


brush  becomes  a  negative  one.  The  currents  of  the  right  quarter 
flow  from  the  right  upper  brush  towards  the  right  lower  cne. 
Hence  the  latter,  too,  is  a  positive  brush.  The  currents  of  the 
lower  quarter  flow  from  the  left  lower  brush  towards  the  right 
lower  one.  Finally,  the  currents  of  the  left  quarter  flow  from  the 
left  lower  brush  towards  the  left  upper  one.  Hence  every  two 
opposite  brushes  are  of  the  same  polarity.  If  we  now  connect  two 
opposite,  corresponding  brushes  with  each  other,  by  a  metal  bridge, 
such  as,  for  instance,  a  bent  copper  strip,  then  we  may  connect  the 
mains  with  any  point  of  the  two  bridges  (see  Fig.  119).  The  E.M.F 


THE  CONTINUOUS  CURRENT  DYNAMO 


111 


FIG.  119. — Brush  Arrangement  of  4-Pole 
Parallel  Armature. 


of  the  armature  is  produced  here  from  one-fourth  of  the  windings 
being  in  series,  and  connecting  these  four  quarters  in  parallel.  Hence 
the  main  current  is  four 
times  as  great  as  the  current 
flowing  in  each  armature 
conductor.  Such  a  winding 
is  called  a  parallel  winding. 
It  is  very  suitable  for  com- 
paratively low  voltages  and 
large  currents,  for  we  may 
even  with  large  currents 
employ  relatively  thin  wires, 
since  each  of  the  conductors 
has  to  carry  the  fourth  part 
of  the  main  current  only, 
and  not  the  half  of  it,  as  was 
the  case  with  the  2  -  pole 
armature.  For  a  6-,  8-,  10- 
pole  machine,  6,  8,  10  brush- 
holders  are  required,  the  cur- 
rent flowing  in  every  arma- 
ture conductor  being  equal 
to  the  6th,  8th,  10th  part  of 

the  main  current  respectively.  The  E.M.F.  of  the  armature  is 
produced  by  connecting  in  series  the  6th,  8th;  10th  part  of  all 
conductors. 

For  higher  voltages,  however,  this  winding  would  require,  for 
the  reason  just  mentioned,  a  large  number  of  windings.  In  these 
cases  it  is  preferred  to  wind  the  armature  so  as  to  get,  similarly 
to  the  2-pole  armature,  but  two  parallel  connected  halves.  The 
E.M.F.  is  then  produced  by  connecting  in  series  the  half  of  all  wind- 
ings. For  this  purpose  it  is  necessary  to  connect  the  single  sections 
of  the  winding  not  directly  with  their  neighbouring  ones,  but,  by 
means  of  copper  bridges,  with  the  opposite  sections,  which  are  always 
under  the  influence  of  a  like  pole.  A  diagram  of  the  connections 
is  given  in  Fig.  120.  The  armature  winding,  shown  in  this  diagram, 
consists  of  fifteen  sections.  Following  the  path  of  the  current,  we 
find  that  it  branches,  after  leaving  the  right  (negative  brush),  one 
part  flowing  through  seven,  the  other  one  through  eight  sections, 
the  electro-motive  forces  of  which  are  added.  The  two  current 
branches  join  again  at  the  second  (positive)  brush.  For  this  winding 
two  brushes  only  are  required,  which  are  distant  from  each  other,  not 
the  half,  but  the  fourth  part  of  the  commutator  circumference.  This 
is  called  series  winding. 

If  a  drum  winding  is  used  for  multipolar  dynamos,  its  pitch  has 
naturally  not  to  be  equal  to  about  the  half  of  the  total  number  of 


112 


ELECTRICAL   ENGINEERING 


armature  wires,  but  has  in  the  case  of  a  4-pole  machine  to  be  equal 
to  about  one-fourth,  with  a  6-pole  machine  to  about  one-sixth  of 
the  total  number  of  wires.  Then  a  proper  series  connection  of  the 
electro-motive  forces  of  the  single  wires  will  be  secured.  By  the 
selection  of  corresponding  pitches,  we  may,  as  with  the  ring  armature, 
get  either  a  series  or  a  parallel  connection  of  the  armature.  In  Fig. 
121  —in  which  the  front  connections  are  marked  by  double  bent  lines 
inside,  and  the  back  connections  by  single  bent  lines  outside  the 


FIG.  120. — Four-Pole  Series  Ring  Armature. 


armature — the  diagram  of  connections  of  a  4-pole  armature,  having 
26  wires,  is  given.  The  pitch  is  7  forwards  and  5  backwards,  and  so 
that,  at  the  front,  wire  1  is  connected  with  wire  8,  and  then,  going 
backwards,  wire  8  is  connected  with  wire  3  at  the  back,  at  the  front 
3  with  10,  at  the  back  10  with  5,  and  so  on.  In  addition  to  being 
denoted  as  parallel  winding,  this  is  also  called  loop  winding.  On 
marking  the  brushes  at  4  points,  \  of  the  commutator  circumference 
distant  from  each  other,  we  find  that  there  are  four  parallel  circuits, 
which  consist  of  nearly  equal  numbers  of  series-connected  wires  that 
are  effective  in  producing  voltage. 


THE  CONTINUOUS  CURRENT  DYNAMO 


113 


Next  let  us  consider  the  diagram  shown  in  Fig.  122.  With  this 
winding  we  do  not  go  backwards  in  making  the  connections  at  the 
back,  but  always  proceed  in  one  direction.  Thus  at  the  front,  wire 
1  is  connected  with  8,  at  the  back  8  with  13,  at  the  front  13  with  20, 
at  the  back  20  with  25,  and  so  on.  In  following  the  course  of  the 
current  we  find,  starting  from  the  positive  brush  (there  are  two 
collecting  places  only  required  with  this  winding),  that  there  are 
two  ways  to  reach  the  negative  brush.  Each  of  these  ways  comprises 


FIG.  121. — Four-Pole  Drum  Parallel  Armature. 

half  of  all  the  armature  windings;  we  have  therefore  two  circuits  con- 
nected in  parallel.  This  is  called  series  or  wave  winding.  Instead 
of  two,  we  may  employ  four  brush-holder  pins.  In  this  case,  two 
opposite  ones  are  of  the  same  polarity,  and  there  is  between  them 
but  one  winding,  consisting  of  neutral  wires.  This  winding  is  short- 
circuited  by  two  brushes;  but  that  is  of  no  disadvantage. 

It  is,  of  course,  impossible    to  deal    here  with  all  the  winding 
combinations  which  can  be  made  by  employing  various  pitches  for 


114  ELECTRICAL   ENGINEERING 

multipolar  machines.    The  few  most  important  types  of  windings 
which  have  been  described  above  will  suffice. 

The  arrangements  of  the  winding  on  a  drum  armature  may  vary  in 
very  many  ways.  With  small  armatures  the  connecting  wires  at  both 
armature  ends  may  be  wound  one  upon  another,  in  a  manner  forming 
a  ball.  Generally,  however,  the  wires  are  arranged  regularly,  side  by 
side.  This  type  of  winding  is  used  in  nearly  all  cases  in  which,  on 
account  of  the  large  cross-sectional  area,  copper  bars  instead  of  wires 


FIG.    122. — Four-Pole  Drum  Series  Armature. 


are  employed.  Frequently  the  connections  between  the  single  con- 
ductors are  made  by  V-shaped  copper  strips,  which  are  joined  together 
with  the  conductors.  Figs.  123  and  126  show  a  type  of  winding 
which  is  suitable  for  armatures,  having  thinner  wires.  The  latter 
are  bent  over  wooden  formers,  and  then  placed  in  position  on  the 
armature.  These  are  called  former  wound. 

From  a  glance  at  the  figures,  it  may  be  seen  that  the  pitch  is  in  Figs. 


THE  CONTINUOUS  CURRENT  DYNAMO 


115 


123  and  126  smaller  than  one-half  of  the  circumference  of  the  respec- 
tive armatures,  and  the  latter  therefore  belong  to  multipolar  machines. 
The  conditions  to  have  in  mind  with  parallel  armatures  of  the  drum 
type,  which  type  is  the  usual  one  in  America,  is  that  the  total  number 
of  conductors,  counting  each  side  of  an  armature  coil  as  a  conductor r 
should  be  even,  and  that  the  pitch  front  and  back  should  be  odd, 


FIG.   123. — Former-wound  Armature  in  course  of  construction. 

differing  by  2.  Instead  of  being  wound,  as  in  Fig.  121,  it  is  cus- 
tomary to  have  two  layers,  the  odd  numbers  being  on  top  and  the 
even  underneath,  the  location  being  as  shown  in  Fig.  81  (p.  82). 
Where  the  armature  core  has  slots,  several  conductors  can  be  put  in 
a  slot.  It  is  only  necessary  that  the  number  of  conductors  be  a 
multiple  of  the  number  of  slots.  A  very  common  arrangement  in 
dynamos  is  to  have  four  conductors  per  slot,  two  on  top  of  the  other 
two.  Since  the  pitch  is  odd  both  front  and  back  and  differs  by  2, 
the  average  pitch  is  even.  For  winding  series  armatures  a  formula 


116  ELECTRICAL  ENGINEERING 

is  convenient.  If  N  equals  the  number  of  conductors  on  the  armature, 
and  if  y  equals  the  pitch  and  p  equals  the  number  of  poles,  then 
N  =  pi/+  or  —  2.  y  may  be  different  at  front  or  back,  but  must  be 
odd  in  each  case.  If  same  at  front  and  back,  y  must  be  odd.  If 
different,  the  average  pitch  may  be  even. 

Multiple  windings  are  used  on  large  apparatus,  uusally  above 
150  Kw.      Series  windings  above  150  Kw.  sometimes   give  trouble 


FIG.  124. — Multiple-formed  Armature  Coil. 

from  unequal  drawing  off  of  current  from  the  brushes  in  the  various 
studs.  Theoretically,  a  series  winding  is  independent  of  air-gap 
variations  when  a  multiple  must  be  uniform  in  this  respect,  as  all 


FIG.  125. — Series-formed  Armature  Coil. 

the  windings,  being  in  multiple,  must  have  equal  voltages,  else  cross- 
currents will  tend  to  occur  or  the  various  parts  of  the  armature  will 
do  different  amounts  of  work.  The  choice  of  series  or  multiple  wind- 
ings is  one  the  designer  has  to  carefully  consider.  Fig.  124  shows 


THE  CONTINUOUS  CURRENT  DYNAMO 


117 


a  formed  armature  coil  suitable  for  a  multiple  wound  armature,  and 
Fig.   125  shows  a  formed  coil  suitable  for  a  series-wound  armature. 


FIG.  126.  — Former-wound  Armature. 


It  will  be  noticed  that  in  the  multiple  armature  the  leads  come  out 
near  together  for  convenience  of  connecting  to  commutator.  In  the 
coil  for  series  connected  armature  the  leads  come  out  apart. 


118 


ELECTRICAL  ENGINEERING 


Sparking  and  Displacement  of  Brushes 

If  the  brushes  on  the  commutator  of  a  dynamo  do  not  occupy 
their  proper  position,  we  may  observe  a  sparking  or  flashing  at  the 
brushes.  Sparking  makes  the  commutator  rough,  and  spoils  the 
latter  as  well  as  the  brushes.  By  carefully  displacing  the  brush- 
rocker,  we  may  easily  find  out  a  position  for  the  brushes  where  the 
sparking  ceases.  If  now,  after  stopping  the  machine,  we  examine 
to  which  armature  wires  those  commutator-bars  are  connected  which 
are  under  the  properly  adjusted  brushes,  we  find  that  these  are  situated 
in  the  neutral  zone,  i.e.  in  the  space  between  the  two  poles.  They 


are  not  exactly  in  the  middle  of  the  neutral  zone,  but  generally  dis- 
placed a  little  forwards  in  the  direction  of  rotation. 

We  are  aware,  that  when  a  circuit  is  broken,  sparking  occurs. 
Let  us  consider  any  winding  of  the  2-pole  ring  armature  (Fig.  127), 
then  we  find  that  through  this  winding,  as  long  as  it  is  on  the  right 
side  of  the  armature,  a  current  is  flowing  in  a  certain  direction 
(marked  by  a  dot).  If  then,  on  the  rotation  of  the  armature,  the 
winding  we  are  discussing  passes  the  lower  brush,  the  direction  of 
the  current  is  altered,  for  the  current  is  flowing  in  the  sense  of  the 
cross  in  the  left  side  of  the  armature.  The  brush  is  always  wide 


THE  CONTINUOUS  CURRENT  DYNAMO  119 

enough  to  touch  two  commutator-bars  simultaneously;  hence  the 
winding  remains  short-circuited  as  long  as  the  brush  touches  at  one 
time  the  two  bars  with  which  the  ends  of  the  windings  are  connected. 
During  this  brief  period  the  winding  belongs  neither  to  the  left-  nor 
to  the  right-hand  armature  circuit.  But  the  current  in  the  short- 
circuited  winding  does  not  stop  suddenly.  A  car  which  is  unlinked 
from  a  running  train  does  not  stop  suddenly,  but  follows  a  certain 
distance.  Similarly,  in  the  short-circuited  winding  the  current  flows 
for  a  certain,  although  it  may  be  a  very  short,  time  in  the  same 
direction  as  before,  and  then  decreases  gradually  to  nothing.  If 
during  this  period  the  armature  be  moved  so  much  that  the  two  bars 
connected  with  the  ends  of  the  winding  are  no  longer  covered  by  the 
brush,  the  current  is  interrupted,  and  there  will  suddenly  flow  through 
the  winding  a  current  in  the  opposite  direction,  i.e.  in  that  of  the 
current  flowing  in  the  wires  on  the  left  armature  half,  so  that  sparking 
would  appear.  To  prevent  this,  the  current  must  be  brought  down  to 
nothing  whilst  the  winding  is  still  short-circuited;  and  immediately 
afterwards,  but  yet  whilst  the  winding  is  short-circuited,  a  current 
must  be  induced  in  the  latter  which  is  in  the  same  direction  as  the 
current  which  will  flow  through  the  winding  during  its  movement  on 
the  left  side.  Then  there  is  no  sudden  change  of  the  current  direction, 
and  thus  no  reason  for  sparking.  We  may  bring  this  about  by  moving 
the  brushes  from  the  middle  of  the  neutral  zone  a  little  forwards  io 
the  direction  of  the  armature  rotation.  In  this  case  the  influence 
of  the  forward  pole  induces  in  the  short-circuited  winding  a  small 
E.M.F.,  but  which  is  just  sufficient  to  destroy  the  current  in  the  coil. 

If  the  armature  current  of  a  given  machine  is  but  small,  then 
we  have  to  displace  the  brushes  a  very  little  from  the  middle 
of  the  neutral  zone  forwards.  If,  however,  the  armature  current 
is  a  large  one,  for  its  sparkless  collection  a  greater  influence  of  the 
forward  pole,  and  hence  a  further  displacement  of  the  brushes,  is 
required.  As  a  rule,  we  may  note  that,  with  an  increasing  load  on 
a  dynamo  the  brush-rocker  is  to  be  moved  forwards  in  the  direction 
of  rotation,  and  with  a  decreasing  load  the  rocker  is  to  be  moved 
backwards. 

When  the  brushes  are  at  the  neutral  point  it  will  be  noticed  (see 
Fig.  127)  that  the  magnetic  influence  of  the  armature  is  as  shown 
by  the  dotted  lines  ABC,  A'B'C';  the  armature  thus  strengthening 
the  pole  tips  at  A  and  B'  and  weakening  those  at  A'  and  B,  since  the 
increase  of  ampere  turns  due  to  increase  of  density  at  two  of  the  pole 
tips  is  greater  than  the  decrease  of  ampere  turns  due  to  lessening  the 
density  at  the  other  two  pole  tips  (which  follows  from  the  shape  of 
the  saturation  curve  as  shown  in  Fig.  91).  The  result  of  this  dis- 
tortion is  to  add  a  necessity  for  increased  ampere  turns  by  the  spools 
of  the  dynamo.  Thus  the  less  the  distortion  the  better  the  regulation 
of  the  machine.  The  wires  between  the  pole  tips  have  no  influence, 
as  the  current  flows  one  way  in  half  of  them,  neutralizing  those  in  the 


120  ELECTRICAL  ENGINEERING 

other  half.  (This  latter  effect  is  not  shown  in  Fig.  127,  since  in  this 
figure  the  brushes  have  a  forward  shift.)  If,  however,  the  brushes 
have  a  forward  shift,  as  in  Fig.  127,  tho  strengthening  and  weakening 
of  pole  tips  occurs  as  just  described,  but  in  addition  the  wires  between 
pole  tips  now  act  to  actually  demagnetize,  thus  pulling  down  the  volt- 
age. Thus,  shifting  the  brushes  tends  to  make  the  dynamo  less  excel- 
lent in  regulating  properties.  Looking  at  Fig.  127,  just  under  the  arrow 
and  similarly  below,  the  turns  oppose  the  flow  of  flux.  The  reader 
can  check  this  by  remembering  the  rule  of  the  production  of  mag- 
netism from  currents.  In  modern  dynamos  of  say  500  Kw.,  these 
cross  ampere  turns  may  be  6000  per  pole.  The  gap  density  at  the 
pole  face  may  be  60,000  per  square  inch,  and  the  back  ampere  turns 
between  pole  tips  due  to  the  load  of  the  brushes  may  be  1800.  The 
total  ampere  turns  required  by  the  magnetic  circuit,  including  gap, 
teeth,  magnet  yoke,  armature  core,  back  ampere  turns,  may  be  50,000. 
In  many  cases  it  is  of  great  advantage  to  employ  carbon  instead 
of  copper  as  the  material  for  dynamo-brushes.  Since  the  contact 


FIG.  128.  FIG.  129. 


Carbon 
Brush-h3lder.  Sliding  Type  Carbon-holder.       Swivel  Type  Carbon-holder. 

resistance  of  carbon  on  the  commutator  is  comparatively  high,  the 
coils  of  the  armature  are  not  directly  short-circuited,  whilst  the  bars, 
connected  with  the  ends  of  the  coil,  are  simultaneously  in  contact 
with  the  brush.  Thus,  with  a  wrong  position  of  the  brushes,  the 
current  produced  in  the  short-circuited  winding  cannot  become  so 
large  as  with  copper  brushes,  and  therefore  the  " commutation"  is  a 
more  gradual  one  with  carbon  than  with  copper  brushes.  Hence 
it  is  no  disadvantage  with  carbon  brushes  if  they  are  rather  wide 
and  touch  more  than  two  bars  at  one  time.  Several  of  the  machines 
shown  in  the  illustrations  are  provided  with  carbon  brushes.  Some 
types  of  carbon  and  other  brush-holders  are  shown  in  Figs.  128-130. 

Carbon  brushes  cannot,  however,  be  employed  in  all  cases.  To 
prevent  the  brushes  from  getting  too  hot,  their  number  and  contact- 
surface  have  generally  to  be  larger  than  with  copper  brushes;  and 
in  many  cases  it  is  therefore  impossible,  with  machines  originally 
designed  for  copper  brushes,  to  afterwards  furnish  them  with  carbon 


THE  CONTINUOUS  CURRENT  DYNAMO         121 

brushes,  because  the  commutator  has  usually  not  the  width  required 
for  this  purpose.  Nowadays  very  many  machines  are  provided  from 
the  first  with  carbon  brushes,  and  these  machines  are  generally  less 
sensitive  with  regard  to  changes  of  load  than  machines  furnished  with 


FIG.  1°0. — Copper  Gauze  Brush-Holder.  FIG.  131. — Brush-Holder  with 

Metal  and  Carbon  Brushes. 


copper  brushes.     Sometimes   a   combination  of  carbon  and  copper 
brushes  is  employed  (Fig.  131). 

There  are  many  modern  machines  not  requiring  a  displacement 
of  brushes  at  all,  and  which,  nevertheless,  run  at  all  loads  practically 
without  sparking.  The  brush-rocker  in  this  case  must  not  be  moved, 
if  cnce  adjusted  properly.  Such  machines  are  said  to  have  a  fixed 
lead. 


Methods  for  changing  Direction  of  Rotation 

We  have  learned,  in  the  chapter  about  self -excitation,  that  we 
must  connect  the  magnet  coils  with  the  armature  brushes  in  a  par- 
ticular way.  so  that  the  armature  may  send  a  current  through  the 
'*  magnet  coils  in  such  a  direction  as  to  strengthen  the  magnetism. 

Let  us  imagine  a  machine  excited  separately  (see  Fig.  132).  By 
connecting  the  upper  end,  III.,  of  the  magnet  coil  with  the  positive, 
rnd  the  lower  end,  IV.,  with  the  negative  pole  of  the  outer  source  of 
current,  the  latter  may  flow  through  the  coil  in  the  direction  shown  in 
the  diagram.  If  now  we  turn  the  armature  towards  the  right,  then 
brush  I.  may,  for  instance,  become  a  positive,  and  brush  II.  a  negative 
one.  (Whether  that  is  really  the  case  or  not,  does  not  depend  on  the 
direction  of  rotation  only,  but  also  on  the  direction  of  winding  of  the 
armature  coils.)  In  altering  the  connections  of  the  machine  for 
self-excitation,  we  have,  naturally,  to  connect  magnet  terminal  III., 


122 


ELECTRICAL   ENGINEERING 


which  was  previously  connected  with  the  positive  battery  pole,  now 
with  the  positive  armature  brush  I.;  and  terminal  IV.,  in  connection 


FIG.  132. — Separately  excited 
Machine — Clockwise  rotation. 


FIG.   133. — Shunt  Dynamo — Clockwise 
rotation. 


with  the  negative  battery  pole  before,  with  the  negative  armature 
"brush  II.,  now.  If,  in  addition,  we  use  the  necessary  regulating 
resistance,  we  get  the  scheme  of  Fig.  133. 

Now   let   us    reverse   the   direction    of   the   armature   rotation^ 


FIG.    134. — Separately  ex-        FIG.  135. — Shunt  Dynamo — Counter-clockwise 
cited  Dynamo — Counter-  rotation, 

clockwise  rotation. 


Then  brush  I.,  which  was  positive  before,  becomes  now  negative 


THE  CONTINUOUS  CURRENT  DYNAMO 


123 


and  brush  II.   becomes  positive   (Fig.    134).     If,  now,  we  connect 
terminal  III.  with  I.,  and  IV.  with  II.,  as  before,  then  the  machine 


FIG.    136 — Series  Dynamo — 
Clockwise  rotation. 


FIG.  137. — Series  Dynamo — Counter- 
clockwise rotation. 


loses  its  magnetism  immediately,  for  the  current  in  the  magnets 
flows   in   the   opposite    direction.        Hence,    if   we   want   the   ma- 


FIG.  138. — Compound  Dynamo—         FIG.  139. — Compound  Dynamo — 
Clockwise   rotation.  Counter-clockwise  rotation. 


chine    to    excite    itself,    we   have  to  connect  terminal   III.  of  the 


124  ELECTRICAL  ENGINEERING 

magnets  with  brush  II.,  and  terminal  IV.  with  brush  I.  (see  Fie;. 
135). 

As  with  shunt  machines,  so  also  with  series  machines,  the  con- 
nections must  be  altered  for  different  directions  of  rotation  of  the 
armature.  This  will  be  readily  understood  without  further  explana- 
tion. The  diagrams  of  connection  for  running  a  series  dynamo 
clockwise  and  counter-clockwise  respectively  are  shown  in  Figs 
136  and  137. 

With  compound  machines,  both  the  shunt  and  the  series  coil 
connections  have  to  be  altered.  Figs.  138  and  139  show  the 
respective  schemes. 

Although  the  left-hand  brush  is  positive  in  the  examples  given  in 
Figs.  132-139,  it  must  not  be  supposed  that,  with  a  clockwise 
rotation,  this  is  always  the  case. 


Causes  of  the  Non-excitation  of  Dynamos 

The  cause  of  a  dynamo  not  exciting  itself  may  very  often  be 
found  in  a  wrong  connection,  i.e.  in  one  for  the  opposite  direction 
of  rotation.  In  this  case,  the  magnet  terminals  have  to  be  changed, 
as  shown  in  the  previous  paragraphs. 

With  some  types  of  dynamos,  more  especially  with  multipolar 
machines,  we  may,  instead  of  changing  the  magnet  connections, 
get  the  same  effect  by  moving  the  brush-rocker.  With  4-pole 
machines  we  have  to  remove  the  brush-rocker  one-fourth,  with 
6-pole  machines  one-sixth  of  the  circumference.  With  2-pole 
machines  this  displacement  of  the  brushes  is  not  usual,  as  in  this  case 
it  would  be  equal  to  one-half  of  the  circumference.  Imagining  the 
brush-rocker  in  Fig.  132  removed  one  half  turn,  so  that  brush  I., 
which  was  before  in  the  neutral  zone  to  the  left,  comes  now  in  the 
neutral  zone  to  the  right  (provided  that  the  cables,  forming  the 
connections  between  the  terminals  and  the  brush-holders,  are  of 
sufficient  length  to  allow  this  turning),  it  is  clear  that  we  get  exactly 
the  same  effect  by  displacing  the  brush-rocker  as  by  changing  the 
magnet  connections  as  in  Fig.  134. 

There  may  also  be  other  reasons  for  a  machine  failing  to  excite. 
In  some  cases  the  speed  of  the  dynamo  may,  for  instance,  be  too 
small.  To  be  clear  about  this,  let  us  consider  the  following  case. 
Suppose  that  the  magnets  of  a  given  dynamo  had  to  be  excited  sepa- 
rately with  90  volts,  in  order  to  get,  at  the  normal  speed,  an  armature 
voltage  of  110.  (As  we  know  in  the  case  of  self-excitation}  the 
remaining  20  volts  would  be  consumed  by  the  shunt  regulating 
resistance.)  If  this  machine  be  run,  not  with  its  full  speed,  but 
only  with  two-thirds  of  it,  the  armature  voltage  would  be  equal  to 


THE  CONTINUOUS  CURRENT  DYNAMO         125 

two-thirds  of  110,  or  about  73  volts.  If,  now,  we  switched  over  the 
machine,  from  separate  to  self-excitation,  after  a  very  short  time  the 
machine  would  lose  its  voltage;  for,  since  the  armature  voltage  is 
smaller  than  that  required  for  the  proper  excitation  of  the  magnets, 
the  strength  of  magnetism  will  soon  be  decreased,  and  the  armature 
voltage  will  become  still  smaller,  the  smaller  voltage  will  again  cause 
a  weakening  of  the  magnetism,  and  so  on. 

Exactly  the  same  may  happen  at  the  proper  speed  of  a  dynamo,  if 
too  large  a  resistance  is  connected  in  series  with  the  shunt  coils.  Thus, 
on  starting  a  dynamo,  the  regulating  resistance  is  short-  or  nearly 
short-circuited,  and,  after  observing  that  the  machine  has  started 
exciting  itself,  the  resistance  is  gradually  switched  in. 

Sometimes  the  resistance  may  also  be  increased  by  a  bad  contact 
between  brushes  and  commutator.  This  may  especially  happen  with 
carbon  brushes,  if  they  are  not  made  to  fit  exactly  the  curved 
surface  of  the  commutator,  thus  making  contact  with  the  commutator 
at  a  few  points  only.  In  this  case,  the  contact  resistance  may  be  a 
very  considerable  one.  But  this  may  easily  be  remedied  by  grinding 
the  carbon  brushes  so  as  to  make  them  fit  the  surface  of  the  com- 
mutator, and  polishing  the  latter  a  little  with  emery. 

Non-excitation  of  a  dynamo  may  further  happen,  if  the  brushes 
are  not  in  the  neutral  zone.  If  an  armature  is  rotating,  it  is  not 
always  possible  to  find  out  the  neutral  zone  of  its  commutator,  for, 
very  often,  and  especially  with  drum  armatures,  the  armature  wires 
are  not  led  straight  to  the  commutator,  but  the  connecting  wires  are 
displaced  by  a  certain  part  of  the  circumference — say,  for  instance,  J,  J, 
and  so  on.  To  be  convinced  that  we  have  the  proper  position  of  the 
brushes,  we  have,  therefore,  to  examine  with  which  commutator-bar 
the  wires  in  the  neutral  zone  are  connected.  Non-excitation  may 
also  be  accounted  for  by  the  loss  of  the  residual  magnetism.  This 
may  sometimes  happen,  if  the  polarity  of  the  machine  has  been 
changed  by  any  accident.  In  such  a  case  sufficient  magnetism  may 
be  regained  by  sending  a  current  from  a  galvanic  battery  through 
the  magnet  coils. 

Finally,  if  there  is  any  fault  with  the  connections,  or  any 
disconnection,  either  in  the  magnet  or  in  the  armature  coils, 
excitation  will  be  prevented.  There  might,  for  instance,  be  con- 
sequent magnet  coils  connected  so  as  to  give  poles  of  the  same  name 
side  by  side,  instead  of  different  poles  alternating.  By  carefully 
following  the  beginnings  and  the  ends  of  the  coils,  such  a  fault  may 
easily  be  found  out.  If  there  is  any  source  of  current  available,  this 
test  may  easily  be  made  by  sending  a  current  through  the  coils,  and 
examining,  by  means  of  a  magnetic  needle,  whether  the  neighbouring 
poles  are  or  are  not  of  the  same  polarity. 

Breaks  in  a  circuit  may  sometimes  be  evident  to  the  eye,  but  in 
other  cases  can  only  be  found  out  by  the  electric  current.  If  we 


126  ELECTRICAL  ENGINEERING 

suppose  a  magnet  coil  to  have  a  disconnection,  then,  by  connecting 


FIG.  140. 


the  magnet  terminals  with  the  two  ends  of  a  source  of  current, 
no  current  will  flow  through  this  circuit  (see  Fig.  140).     If  now  we 

connect  one  pole  of  a  glow  lamp,  the 
terminals  of  which  terminate  in  two 
long  wires  (see  Fig.  141) ,  with  say  the 
positive  pole  of  the  source  of  current, 
and  touch  with  the  other  pole  of  the 
lamp  the  terminals  of  the  magnet 
coils  successively,  we  shall  observe 
the  following:  On  touching  the  mag- 
net terminal  a,  the  lamp  will  glow, 
as  there  is  a  connection  between 
terminal  a  and  the  second  pole  of 
the  source  of  current.  The  same  will 
be  the  case  if  we  connect  the  terminals 
b,  c,  d,  e,  respectively,  as  they  are  all 
in  connection  with  the  negative  pole. 
But  if,  further,  we  come  to  /,  the 
lamp  will  no  longer  glow,  showing  that  there  is  a  disconnection 
between  e  and  /.  For  /  is  not  in  connection  with  the  positive,  but 


FIG.  141. — Testing  Lamp. 


THE  CONTINUOUS  CURRENT  DYNAMO         127 

only  with  the  negative  pole,  and,  if  we  connect  both  terminals  of  a 
lamp  with  the  same  pole,  it  can,  of  course,  not  glow. 

A  lamp  of  suitable  voltage  should  be  used  for  the  test.  When 
the  voltage  is  high  we  can  employ  several  lamps  connected  in  series, 
instead  of  one  lamp. 

With  machines  that  are  not  too  small,  and  with  a  suitable  voltage 
of  the  outer  source  of  current,  a  lamp  serves  as  an  excellent  means 
for  finding  out  disconnections  of  coils.  Instead  of  a  lamp,  we  can 
with  greater  certainty  use  a  voltmeter.  As  long  as  the  two  ends  of 
the  voltmeter  are  on  different  sides  of  the  point  of  disconnection, 
a  deflection  of  the  needle  can  be  observed.  The  latter  indicates  the 
full  voltage.  But  as  soon  as  the  point  of  disconnection  is  passed 
over,  so  that  both  terminals  of  the  voltmeter  are  connected  with  one 
side  of  the  point  of  disconnection  only,  the  voltmeter  needle  stops 
at  zero. 

The  magneto  used  for  insulation  tests  (p.  73)  may  also  be 
employed  for  finding  out  the  point  of  disconnection. 


Automatic  Shunt  Regulator 


A  compound  winding  can  keep  constant  the  voltage  of  a  dynamo, 
provided  that  the  latter  is  running  with  a  constant  speed.  As  some- 
times the  speed  of  the  driving  engine — such  as,  for  instance,  the 
steam-engine — varies  according  to  the  variation  of  the  load,  and, 
further,  since  the  compound  winding  is  not  useful  for  all  machines, 
automatic  shunt  regulators  are  employed  whenever  a  constant, 
pressure  is  required,  without  having  a  man  always  present  to  alter 
the  shunt  regulator  as  necessary.  A  simple  construction  for  such 
an  apparatus  is  shown  in  Fig.  142.  The  wire  spirals,  forming  the 
regulating  resistance,  are  arranged  on  an  iron  frame  and  supported 
by  porcelain  insulators  in  the  usual  way.  The  wires  coming  from 
the  ends  of  the  single  spirals  are  not  connected  with  the  usual, 
circularly  arranged  contact  pieces,  but  are  fixed  vertically  side  by 
side,  and  cut  to  different  lengths.  These  wires  dip  into  a  movable 
glass  vessel,  filled  with  mercury,  and  fixed  on  the  top  of  an  iron  core. 
The  latter  is  suspended  on  one  arm  of  a  lever,  and  kept  in  balance 
by  a  weight  fixed  on  the  other  arm  of  the  lever.  With  its  lower 
end  the  iron  core  dips  into  a  fixed  coil,  consisting  of  very  fine  wire, 
the  ends  of  which  are  switched  on  to  the  full  dynamo  voltage.  If 
the  dynamo  pressure  falls  (for  instance,  through  slower  running  of  the 
dynamo),  the  current  flowing  through  the  coil  decreases,  the  iron  core 


128 


ELECTRICAL   ENGINEERING 


is  therefore  less  attracted  than  before,  and  the  counterweight  is  able 
to  lift  the  iron  core  a  little,  so  that  the  glass  vessel,  and  with  it  the 
level  of  the  mercury,  is  raised,  touching  some  of  the  graduated  wires, 
and  the  respective  resistance  coils  are  short-circuited  by  the  mercury. 
Hence  less  resistance  is  now  connected  in  series  with  the  shunt,  and 
the  voltage  of  the  machine  can  rise  to  its  proper  value. 

If,  on  the  other  hand,  the  voltage  of  the  machine  grows  too  great, 


FIG.  142. — Automatic  Shunt  Regulator  (Voigt  &  Hdffner). 


the  current  flowing  through  the  coil  will  also  become  greater.  The 
iron  core  will  then  be  pulled  down  a  little,  and  the  vessel  with 
the  mercury  is  lowered,  causing  resistance  to  be  again  included  in 
the  shunt  circuit,  the  shunt  current  therefore  decreases,  causing  the 
dynamo  voltage  to  be  brought  to  its  normal  value. 

There  are,  besides  the  apparatus  described,  many  other  ingenious 
constructions  of  automatic  shunt  regulators,  but  which  we  cannot 
deal  with  here. 


THE  CONTINUOUS  CURRENT  DYNAMO         129 


Efficiency  of  Dynamos 

With  every  kind  of  work  we  have  also  to  do  things  that  are 
-useless,  in  order  to  get  the  intended  effect.  For  example,  to  convey 
people  or  goods,  it  is  also  necessary  to  convey  the  carriage  which  is 
employed  for  the  conveyance.  The  work  which  has  to  be  spent  in  con- 
veying the  carriage  represents,  in  this  case,  in  which  the  main  pur- 
pose is  the  conveyance  of  the  people  and  goods  only,  a  loss  of  mechani- 
cal energy.  A  stove,  only  intended  to  warm  the  air  of  a  room,  also 
heats  a  great  quantity  of  air  which  does  not  remain  in  the  room,  but 
escapes  up  the  chimney,  hence  causing  a  loss. 

In  like  manner  there  are  losses  in  the  transformation  of  mechani- 
cal into  electrical  energy  by  means  of  a  dynamo.  These  losses  may 
be  classified  as  follows: — 

Firstly,  work  spent  for  excitation  of  the  magnets. 

Secondly,  losses  due  to  the  resistance  of  the  armature-winding. 

Thirdly,  due  to  eddy  currents  and  to  the  varying  magnetism  of 
the  armature  core. 

And  Fourthly,  losses  due  to  mechanical  friction. 

The  current  which  has  to  be  sent  through  the  magnet  coils  of  a 
dynamo,  in  order  to  excite  the  magnets,  is  not  available  in  the  outer 
circuit.  If  with  a  machine,  giving  100  amps,  at  a  voltage  of  110,  the 
shunt  current  were  3  amps.,  then  the  loss  due  to  excitation  would  be 
equal  to  330  watts,  or  3  per  cent,  of  the  total  dynamo  output. 

A  further  loss  depends  on  the  ohmic  resistance  of  the  armature, 
including  the  resistance  of  the  brushes  and  the  connecting  cables. 
If,  in  our  example,  this  resistance  were  0.02o>,  then  the  voltage  drop 
would  be  0.02X100=2  volts,  and  the  loss  2x100=200  watts— that 
is,  nearly  2  per  cent,  of  the  total  output. 

There  are,  further,  as  we  know,  eddy  currents  in  the  armature. 
By  employing  very  thin  iron  discs,  these  eddy  currents  may  be  re- 
duced to  a  very  small  value,  so  that  the  loss  depending  on  them  may 
be  but  1  per  cent.,  or  even  less. 

The  continual  reversal  of  the  magnetism  of  the  armature  iron 
also  involves  a  certain  amount  of  work.  As  we  know,  the  molec- 
ular magnets  of  the  iron  are  not  absolutely  freely  movable,  but  a 
kind  of  friction  has  to  be  overcome  to  turn  the  molecular  magnets 
in  the  direction  of  the  lines  of  force.  For  overcoming  this  resistance, 
however,  a  certain  amount  of  work  is  required,  which  causes — like 
all  other  losses — a  heating  of  the  machine.  This  is  generally  called 
the  hysteresis  loss. 

Finally,  there  is  to  be  considered  the  mechanical  loss  due  to 
friction  in  the  bearings  of  a  dynamo.  These  losses  are,  however, 
not  great,  for  dynamo  bearings  are  generally  very  well  oiled. 


130  ELECTRICAL  ENGINEERING 

Again,  the  air  offers  a  certain  resistance  to  the  rapid  rotation  of  the 
armature,  thus  involving  a  further  small  loss. 

The  total  amount  of  all  these  losses  is  not  considerable. 
With  large  dynamos  it  is  equal  to  about  4  to  6  per  cent.,  with  dynamos 
of  medium  size  10  to  15  per  cent.,  and  with  small  ones  up  to  30  per 
cent,  of  the  total  output.  Thus  we  get  for  mechanical  power,  sup- 
plied to  the  dynamo,  and  corresponding  to  100  watts,  according 
to  the  size  and  excellence  of  the  machine,  96,  90,  85,  70  watts,  re- 
spectively, or  the  efficiency  of  the  dynamo  is  96,  90,  85,  70  per 
cent. 

Perhaps  no  other  machine,  employed  for  transforming  one  kind  of 
energy  into  another,  is  so  efficient  as  a  good  dynamo. 

If  all  the  losses  mentioned  above = L,  and  the  output  of  the  dynamo 

W 

=  W,  then  the  efficiency =-KT  .  j  •  This  is  usually  called  the  com- 
mercial efficiency. 

To  measure  the  efficiency  of  a  dynamo,  the  losses  are  required. 
They  are,  first,  loss  in  the  field  windings.  If  R8=  resistance  of  series 
field,  and  R«A= resistance  of  shunt  field,  the  loss  in  them  can  be  cal- 
culated, since  the  current  flowing  in  them  is  known.  The  loss  equals 
I2R,  when  I  is  the  current  and  R  the  resistance. 

If  Ra = resistance  of  armature  and  R&  the  resistance  of  brush  con- 
tact on  the  commutator,  the  loss  in  copper  of  the  armature  and  in 
contact  brushes  can  be  similarly  calculated.  The  resistance  of  brush 
contact  with  carbon  brushes  is  about  .028  ohm  per  square  inch. 
With  copper  brushes  this  resistance  is  about  .003  ohm  per  square 
inch.  The  density  of  current  with  carbon  brushes  is  usually  from 
30  to -40  amperes  per  square  inch;  with  copper  brushes  about  150 
amperes  per  square  inch. 

There  remains  to  be  found  the  friction  and  hysteresis  loss.  A 
convenient  method  to  obtain  this  is  to  run  the  dynamo  free  until  all 
friction  becomes  uniform.  Put  upon  the  armature  a  voltage  equal 
to  the  operating  voltage  E  at  the  terminals  of  the  machine  plus  the 
voltage  drop  in  the  armature  when  operating  under  full  load.  This 
is  equal  to  E  +  IRa.  The  dynamo  armature  should  then  be  run  at 
full  speed  as  a  motor  by  adjusting  the  field  current.  Under  these 
conditions,  since  so  little  current  would  be  flowing  into  the  armature, 
the  back  E.M.F.  would  be  equal  to  the  applied.  But  this  is  equal 
to  E  +  IRa,  which  is  the  voltage  generated  by  the  machine  when 
operating  as  a  dynamo  at  full  load.  And  since  the  speed  is  sot  by 
adjusting  the  field  current  till  normal  dynamo  speed  is  obtained,  the 
flux  must  be  the  same  as  when  the  machine  is  operating  as  a  dynamo. 
And  thus  the  hysteresis  loss  is  the  same  as  when  running  as  a  dynamo. 
But  the  input  measured  by  reading  the  current  taken,  It,  multiplied 
by  E+IR-o  applied,  measures  all  losses.  Subtracting  from  this  in- 


THE  CONTINUOUS  CURRENT   DYNAMO  131 

put  the  small  amount  of  armature  resistance  loss,  or  Ii2R,  created  by 
the  "  running  light"  current  just  mentioned  (Ij  equals  a  very  small 
percentage  of  the  full  load  armature  current),  leaves  the  hysteresis 
plus  friction  desired,  and  thus  all  the  losses  are  determined.  This- 
is  the  stray  power  method  first  used  by  Dr.  Hopkinson  of  England. 
This  same  method  of  getting  efficiency  can  be  applied  to  a  motor,  but 
in  this  case  the  voltage  to  apply  to  the  commutator  is  E— IRa,  for 
a  motor  when  running  under  load  creates  a  back  E.M.F,  of  this  amount, 

._,  flux  X  external  wires  X  revolutions 

since  E.M.F.-  100,000,000  ~    as  haS    been   pre' 

viously  shown. 

If  the  same  E.M.F.  and  revolutions  are  set,  then  the  flux  must 
come  the  same,  and  if  the  flux  and  speed  are  the  same,  the  hysteresis 
must  be  the  same. 

Another  method  of  measuring  the  iron  loss  or  core  loss  of  a  dynamo, 
as  well  as  the  friction,  is  to  belt  to  the  dynamo  whose  core  loss  is 
desired,  a  motor  of  such  a  size  as  to  be  capable  of  handling  the  load 
conveniently  (say  a  motor  of  10  per  cent,  of  the  size  of  the  dynamo). 
Separately  excite  the  field  of  the  motor  and  keep  it  constant  through- 
out the  test.  Thus  the  driving  motor  iron  losses  will  remain  constant 
as  far  as  the  field  is  concerned.  Apply  enough  voltage  to  the  motor 
armature  to  run  the  dynamo  armature  at  normal  speed,  and  measure 
the  input  to  the  motor  by  multiplying  the  current  taken  by  the  motor 
armature  by  the  voltage  applied.  Repeat  this  reading  with  the  normal 
voltage  in  the  dynamo  whose  core  loss  is  being  measured.  Then 
the  difference  between  these  two  readings  gives  the  loss  due  to  putting 
on  the  field  and  therefore  obtaining  the  normal  voltage  of  the  dynamo. 
To  obtain  the  friction  of  the  dynamo,  subtract  from  the  input  of  the 
driving  motor,  when  running  the  dynamo  without  field  current,  the 
input  of  the  driving  motor  with  the  belt  removed  (which  thus  records 
losses  of  the  driving  motor  itself) ,  and  the  remainder  gives  the  fric- 
tion of  the  dynamo.  Knowing,  therefore,  the  friction  and  the  core 
loss,  and  adding  to  the  other  losses,  the  efficiency  is  calculated  as 
before. 

In  reading  the  input  to  the  motor,  its  speed,  and  that  of  the  motor 
having  core  loss  taken,  is  kept  constant  by  means  of  a  tachometer 
fastened  to  its  shaft  or  to  the  dynamo  shaft.  Also  it  is  proper  for 
a  refinement  to  take  out  from  each  input  reading  to  the  motor  its 
own  PR  of  armature  and  brush  contact,  since  this  value  varies  with 
the  different  inputs,  and,  being  the  only  variable,  its  subtraction 
makes  the  remainder  one  of  pure  input  transferred,  barring  the  varying 
loss  in  the  bearings  due  to  the  varying  belt  pull,  which  may  be  neglected. 
This  last  method  is  a  very  common  method  of  measuring  the  core 
losses  of  dynamo  machinery.  In  taking  the  curve,  the  voltage  on  the 


132  ELECTKICAL  ENGINEERING 

driving  motor  should  be  approximately  constant  throughout  the 
test.  If  it  is  not,  a  slipping  of  the  belt  must  be  occurring,  or  the 
internal  drop  of  armature  or  brush  contact  resistance  must  be  ex- 
cessive. A  driving  motor  fitted  with  copper  brushes  to  reduce  drop 
in  brush  contact  and  only  loaded  under  maximum  conditions  of  core 
loss  on  the  dynamo  to  one-half  load  gives  the  best  results. 


CHAPTER  IV 
THE  ELECTRIC  MOTOR 

IF  we  send  a  current  through  the  armature  of  a  dynamo,  whose 
magnetic  field  is  excited,  the  armature  will  be  put  into  motion. 
This  will  be  at  once  expected  from  our  study  of  the  action  of  the 
Deprez  ammeter.  With  the  dynamo  armature  there  will,  however, 
take  place  not  only  a  single  movement,  but  a  permanent  rotation. 
Owing  to  the  action  of  the  commutator,  the  current  flows  through 
all  wires  on  one-half  of  the  armature,  which  are  under  the  influence 
of  the  north  pole,  in  one  direction,  and  through  all  wires  which  are 
under  the  influence  of  the  south  pole,  in  the  opposite  direction;  hence, 
as  long  as  a  current  from  an  outer  source  is  sent  through  it,  the  arma- 
ture will  rotate.  The  machine  now  absorbs  electrical  and  supplies 
mechanical  energy.  In  this  case  the  machine  is  called  an  electric 
motor  or  an  electro-motor,  which  we  may  speak  of  simply  as  a 
motor;  whereas  a  machine  by  producing  current  is  called  a  dynamo 
or  a  generator. 

The  direction  of  rotation  of  the  armature  in  Fig.  143  may  easily 
be  determined  by  Ampere's  Rule.  The  armature  will  rotate  counter- 
clockwise. 

The  scheme  of  the  motor  armature  (Fig.  143)  is  strictly  in  accord- 
ance with  that  of  the  dynamo  armature  (Fig.  73).  In  both  cases 
the  pole  to  the  left  is  a  north  pole,  and  the  current  in  the  left  half 
of  the  armature  is  directed  from  the  spectator.  We  had  to  turn 
the  dynamo  armature  towards  the  right,  in  order  to  get  a  current  in 
the  direction  marked ;  the  motor  armature  will  run  towards  the  left, 
if  a  current  having  the  same  direction  flows  through  it. 

We  have  seen,  when  considering  the  current  direction  in  a  dy- 
namo armature,  that  in  each  armature  conductor  a  current  is  pro- 
duced which  would  turn  the  armature  in  an  opposite  direction,  if 
there  did  not  exist  any  other  force.  In  this  case  the  induced  cur- 
rents produce  an  internal  force  in  opposition  to  the  external  driving 
force  supplied  to  the  armature. 

With  the  motor  we  find  the  same  action,  but  with  a  remarkable 
difference.  We  know  that  in  each  wire,  rotating  in  a  magnetic  field, 
an  E.M.F.  is  induced.  With  the  electric  motor  we  have  an  armature 

133 


134 


ELECTRICAL  ENGINEERING 


which  rotates  in  a  strong  magnetic  field.  Naturally  it  does  not 
make  any  difference  whether  this  rotation  is  effected  by  an  electric 
current,  or  by  an  outside  driving  force.  In  each  wire  on  rotation  an 
E.M.F.  is  induced.  To  determine  the  direction  of  this  E.M.F.  we 
have  simply  to  compare  this  scheme  with  that  in  Fig.  73,  in  which 


FIG.  143. — Motor  Ring  Armature. 

we  had  the  same  armature  rotating  towards  the  right.  The  direc- 
tion of  the  induced  E.M.F.  was  there  marked  by  dots  and  crosses. 
The  lower  brush  was  positive,  the  upper  one  negative.  But  now 
the  armature  is  rotating  in  the  opposite  direction,  hence  the  direc- 
tion of  the  current  in  the  armature  is  reversed,  the  upper  brush 
becoming  positive,  the  lower  one  negative;  and  we  see,  therefore, 
that  the  E.M.F.  produced  by  the  rotation  of  the  armature  acts  against 
the  current  sent  from  the  source  of  current  into  the  armature.  The 
result  is  that  the  E.M.F.  produced  by  the  rotation  would,  if  no  other 
E.M.F.  existed  in  the  circuit,  cause  a  current  to  flow  in  a  direction 
which  is  opposite  to  that  of  the  current  sent  into  the  armature  by 
the  outer  source.  The  E.M.F.  produced  by  the  armature  of  a  run- 
ning electric  motor  is  therefore  called  the  back  electromotive 
force  or  counter-electromotive  force  of  the  motor.  It  follows  the 
same  law,  of  course,  as  the  dynamo  E.M.F.  and  therefore  equals 

^— — -,  as  has  been  shown.   It  causes  the  current  flowing  through 
100,000,000 

the  armature  to  be  far  smaller  than  we  should  calculate  it  to  be  by 
dividing  the  terminal  voltage  by  the  resistance  of  the  armature. 


THE  ELECTRIC   MOTOR  135 

If,  for  instance,  we  connected  a  stationary  armature,  having 
a  resistance  of  yf^w,  suddenly  with  110  volts,  then,  through  the 
armature,  according  to  Ohm's  Law,  a  current  of  i.H  =  3666  amps, 
would  flow.  This  excessive  current  would  instantly  destroy  the 
armature,  and  melt  both  the  brushes  and  the  mains.  If,  however, 
we  do  not  connect  the  armature  immediately  with  its  full  voltage, 
but  first  interpose  in  series  with  it  a  resistance  of  about  5aj,  then 
a  current  of  a  little  more  than  20  amps,  will  flow  through  the 
armature  and  the  resistance.  The  armature  then  starts  to  rotate, 
and  produces  by  its  rotation  in  the  magnetic  field  a  back  electro- 
motive force,  which  soon  reduces  the  current  to  a  smaller  value. 
The  series  resistance  may  now  be  reduced.  The  motor  will  then  run 
faster,  its  back  electro-motive  force  will  grow,  and,  if  we  gradually 
short-circuit  the  series  resistance,  the  motor  will  reach  its  full 
speed. 

A  simple  consideration  will  show  us  what  this  speed  must  be. 
Obviously  the  motor  will  never  run  so  fast  as  to  produce  a  back 
E.M.F.,  equal  to  the  E.M.F.  of  the  source  of  current,  since  in  this 
case  no  current  would  flow  through  the  armature,  and  it  would  not 
exert  rotary  power.  But  a  certain  amount  of  power  is  required— 
although  it  may  be  quite  small — for  overcoming  the  friction  in  the 
bearings  and  the  resistance  due  to  the  air.  Thus  the  current  can 
never  become  actually  nothing,  but  must,  for  instance,  with  a  motor 
which  is  designed  for  100  amps.,  be  at  no  load  about  3  to  5  amps. 
If  the  outer  E.M.F.  or  terminal  voltage  be  110,  the  back  E.M.F. 
will  not  be  quite  110  volts,  but  at  no  load  nearly  as  much,  viz. 
only  some  tenths  of  a  volt  less  than  110. 

If  now  we  load  the  motor,  for  instance  by  putting  a  brake  on, 
or  by  making  it  drive  a  shaft  by  means  of  a  belt,  the  small  current 
going  through  the  armature  at  no  load  cannot  exert  sufficient  power 
to  overcome  the  load.  Thus  the  motor  speed  will  decrease  a  little. 
But  as  soon  as  the  motor  is  running  a  little  slower,  say  with  990 
instead  of  1000  revolutions,  its  back  E.M.F.  will  decrease  in  the 
same  proportion.  The  balance  of  the  outer  above  the  inner  voltage  is 
therefore  greater,  and  the  armature  current  can  now  grow  to  such  an 
extent  as  to  produce  sufficient  rotary  power  to  counterbalance  the 
load.  The  back  E.M.F.  will  be  in  this  case  about  109  volts.  If  the 
load  be  doubled,  the  motor  will  run  still  slower,  until  its  back  E.M.F. 
falls  to  about  108  volts,  the  remaining  difference  of  about  2  volts 
sending  a  current  double  in  strength  through  the  armature.  If  the 
load  be  removed,  the  motor  will  again  run  faster  until  its  back  E.M.F. 
becomes  nearly  110  volts. 

We  may,  then,  conclude  that  an  electric  motor  regulates  in  a 
perfect  manner  the  absorption  of  electrical  power  according  to  the 
work  to  be  done.  With  steam  engines,  turbines,  etc.,  the  steam  or 
water  supplied  has  to  be  regulated  according  to  the  load  by  means 


136 


ELECTRICAL  ENGINEERING 


of  complicated  governors.     The  electric  motor,  on  the  other  hand,  is 
self-governing. 

The  larger  the  armature  resistance  of  a  motor,  the  greater 
must,  for  a  definite  load,  be  the  difference  between  the  terminal 
voltage  and  back  E.M.F.,  in  order  to  get  the  necessary  current  to  flow 
through  the  armature,  and  the  greater,  therefore,  must  be  the  drop  of 
speed. 


The  Shunt  Motor 

In  the  above  reasoning  we  have  presumed  that  the  magnetic  field 
of  the  motor  is  of  constant  strength.  This  may  be  effected  by  con- 
necting the  magnet  coils  directly  to  the  outer  source  of  pressure.  To 
the  current  two  ways  are  then  offered;  one  through  the  armature, 
and  the  other  through  the  magnet  coils.  The  latter  are  in  shunt  with 
the  armature.  This  motor  is  called  a  shunt  motor.  About  the 
working  of  such  a  motor  we  have  spoken  already.  With  regard  to 
the  speed  of  a  shunt  motor,  we  have  just  learned  that  the  speed 
decreases  with  increasing  load.  This  fall  of  speed  is,  at  a  constant 
voltage,  small.  It  varies  according  to  the  type  of  motor,  being 
from  TV  to  5  per  cent.,  unless  the  motors  are  small,  when  the  variation 
may  be  much  greater.  Practically  speaking,  the  speed  of  a  com- 
mercial shunt  motor  may  be  considered  as  nearly  constant  with 
varying  loads. 

It  is  most  important  to  learn  how  a  shunt  motor  should  be  started. 
To  get  a  proper  start,  the  magnetic  field  has  to  be  fully  excited. 

It  is,  therefore,  necessary  to  switch 
the  magnet  coils  immediately  on  to 
the  voltage  of  supply,  whereas,  as 
we  have  seen,  with  the  armature  a 
resistance  must  be  connected  in 
series  at  starting.  To  get  both 
connections  simultaneously,  starters 
for  shunt  motors  are  constructed 
as  shown  diagrammatically  in  Fig. 
144.  The  centre  of  the  starting 
lever  is  connected  with  one  main. 
The  lever  slides  over  a  row  of 
contacts,  (which  are  connected  with 
the  ends  of  the  resistance  spirals,) 
and  a  slip-ring.  The  latter  is  con- 
nected with  one  end  of  the  magnet 
winding,  the  last  of  the  contact-pieces  (on  the  left)  with  one 
armature  brush.  The  other  brush  and  the  other  end  of  the  magnet 
^winding  are  both  connected  with  the  second  main. 


FIG.  144. — Shunt  Motor  with 
Starting  Resistance. 


THE  ELECTRIC   MOTOR  137 

As  long  as  the  lever  is  in  its  extreme  position  to  the  right  the 
motor  is  at  rest.  Neither  the  slip-ring  nor  the  contact-pieces,  which 
are  in  contact  with  the  resistance  spirals,  are  touched  by  the  lever,  so 
that  the  motor  is  in  connection  with  one  (the  negative)  main  only, 
and,  naturally,  no  current  flows  through  it.  In  moving  the  lever  a 
little  towards  the  left,  it  makes  contact  both  with  the  slip-ring  and 
the  first  resistance  contact.  As  the  slip-ring  is  connected  with  one 
shunt  terminal,  the  magnet  winding  is  immediately  switched  on  the 
full  voltage,  and  the  full  magnetizing  current  flows.  If,  for  instance, 
the  resistance  of  the  shunt  winding  were  55w,  then,  at  a  voltage  of 
110,  the  shunt  current  would  be  2  amps.  Although  the  magnets  in 
this  arrangement  are  fully  excited,  the  armature  is  still  in  series  with 
the  whole  of  the  starting  resistance,  which  may  be  about  5cu.  Through 
the  armature  a  current  of  about  20  amps,  therefore  flows.  It  will 
start  to  rotate,  and,  gradually  the  lever  is  moved  to  its  extreme 
position  to  the  left,  when  finally  the  armature  is  switched  on  to  the 
full  voltage  of  110.  During  the  whole  time  of  starting  the  motor, 
the  magnets  are  fully  excited. 


Speed  Regulation 

The  speed  of  a  motor  depends  on  the  voltage  of  supply  and  the 
strength  of  its  magnetic  field.  As  we  have  learned,  the  motor  always 
attempts  to  rotate  so  fast  as  to  produce  a  back  E.M.F.  nearly  equal 
to  the  terminal  voltage.  Hence,  by  doubling  the  latter,  the  motor 
will  run  with  nearly  double  the  speed.  By  decreasing  the  terminal 
voltage,  we  decrease  the  speed  of  the  motor. 

A  reduction  of  speed  may  therefore  be  effected  by  switching  per- 
manently a  resistance  in  series  with  the  motor,  since,  in  this  case,  the 
armature  voltage  will  no  longer  be  equal  to  the  voltage  of  the  outer 
circuit.  The  series  resistance  will  consume  a  definite  part  of  the 
voltage.  If,  for  instance,  the  motor  speed  were  1000  revolutions  at  a 
voltage  of  110,  then,  if  we  switch  a  resistance  of  leu  in  series  with 
the  armature,  the  terminal  voltage,  and  with  that  the  speed  of  the 
motor,  will  vary  according  to  the  load,  or — what  amounts  to  the 
same  thing — according  to  the  current  strength  required  for  over- 
coming this  load.  If  the  armature  current  were  11  amps.,  in  the 
series  resistance  of  leu  a  voltage  of  11,  i.e.  the  tenth  part  of  the  total 
voltage,  would  be  consumed.  The  motor  wrill  therefore  make  900 
instead  of  1000  revolutions  per  minute.  If,  due  to  an  increasing 
load,  the  armature  current  grows  to  22  amps.,  then  in  the  series 
resistance  22  volts  will  be  consumed.  The  motor  speed  will  fall 


138 


ELECTRICAL   ENGINEERING 


9y     Shunt  Regulator 


down  to  about  800  revolutions  per  minute.  At  a  current  of  55  amps, 
the  speed  will  be  equal  to  about  one-half  of  the  normal  speed.  Thus 
we  can  regulate  the  speed  of  a  motor  by  means  of  a  series  resistance 
when  it  is  required  to  run  below  the  normal  speed. 

Another  way  to  regulate  the  speed  is  to  vary  the  shunt  current. 
If  in  the  magnet  circuit  we  arrange  a  shunt  regulating  resistance 

(see  Fig.  145),  as  we  have  done 
with  the  dynamo,  we  may,  by 
switching  in  some  resistance, 
weaken  the  shunt  current.  Let 
us  now  start  the  motor  and 
short-circuit  the  starter.  To 
produce  in  the  weakened  field 
the  same  back  E.M.F.  as  be- 
fore, the  motor  has,  naturally, 
to  run  much  faster.  Thus,  by 
switching  in  some  resistance  in 
the  shunt  circuit,  the  motor 
speed  may  be  increased  above 
its  normal  value — say,  for  in- 
stance, from  1000  to  1100, 
1200,  and  even  1400  revolutions 
per  minute. 

Care    must,    of    course,    be 

FIG.  145.— Shunt  Motor  with  Starting  taken,  not  to  disconnect  the 
Resistance  and  Shunt  Regulator  for  shunt  circuit  entirely,  whilst 
Speed  Regulation.  the  armature  is  still  in  circuit. 

In  such  a  case  the  strength  of 

the  magnetic  field  would  be  practically  nil,  since  there  would 
only  be  the  weak  residual  magnetism.  Two  things  may  then 
happen.  Either  the  motor  reaches  a  dangerously  high  speed,  in 
order  to  produce  a  sufficient  back  E.M.F.,  with  the  weak  magnetic 
field,  and  in  this  case  the  excessive  speed  may  cause  the  belt- 
pulley,  the  commutator,  or  the  armature  winding  to  burst  into 
pieces;  or,  the  motor  is  prevented  from  reaching  such  a  high  speed 
by  a  heavy  load,  then  it  can  produce  but  a  small  back  E.M.F.,  and 
consequently  a  current  of  so  great  a  magnitude  will  flow  through  the 
armature  as  to  destroy  the  latter  and  melt  the  brushes,  or,  what 
would  be  more  desirable,  to  cause  the  fuses  to  go. 

To  prevent  accidents  of  this  kind,  the  shunt  regulators  for  motors 
are  generally  made  so  as  to  render  a  disconnection  of  the  shunt  circuit 
impossible.  The  latter  can  then  be  switched  off  simultaneously  with 
the  armature -circuit,  by  means  of  the  starting  lever,  but  in  no  other 
way. 

Shunt  motors  are  usually  used  for  power  in  factories,  workshops, 
etc.,  where  constant  speed  under  varying  load  is  desired. 


THE   ELECTRIC  MOTOR 


139 


FIG.    146. —Scries  Motor  with 
Starting  Resistance. 


The  Series  Motor 

Instead  of  exciting  the  magnetic  field  of  a  motor  by  many  wind- 
ings, which  are  switched  directly  on  to  the  terminal  voltage,  this 
may,  similarly  to  the  series  wind- 
ing of  a  dynamo,  also  be  done  by 
providing  the  magnet  coils  of  the 
motor  with  comparatively  few 
turns  of  thick  wire,  connected  in 
series  with  the  motor  armature. 
These  motors  are  called  series 
motors.  A  diagram  of  connections 
for  a  series  motor,  and  the  starter 
belonging  to  it,  is  shown  in  Fig. 
146.  As  can  be  seen  from  the 
diagram,  the  starter  is  simpler  than 
that  of  a  shunt  motor,  on  account 
of  the  omission  of  the  shunt  slip- 
ring. 

The    properties    of    the    series 
motor  are  of  quite  another  kind  to 

those  of  the  shunt  motor.  With  the  latter  we  have  a  magnetic  field 
of  constant  strength,  and  the  speed  of  the  motor  is  practically  con- 
stant at  varying  loads.  With  the  series  motor  the  field  is  stronger 
the  larger  the  armature  current  of  the  motor,  since  the  latter  flows 
through  the  magnet  coils  as  well.  If  the  motor  is  loaded  but  little, 
and  thus  the  armature  current  small,  the  magnetic  field  will  be 
weak.  If  now  the  motor  is  switched  on  to  a  constant  voltage, 
such  as,  for  instance,  the  mains  of  a  lighting  plant,  then  it  must 
run  with  a  very  high  speed,  to  produce  in  the  weak  magnetic  field 
a  back  E.M.F.  corresponding  to  the  outer  voltage.  If,  on  the  other 
hand,  the  motor  is  loaded  very  heavily,  its  magnetic  field  will  also 
be  a  very  strong  one.  Thus  the  speed  at  which  the  motor  pro- 
duces a  back  E.M.F.,  corresponding  to  the  outer  voltage,  will  be 
far  lower  than  before.  A  series  motor  must  never  run  light  or 
without  load,  for  in  this  case  its  field  would  be  very  weak, 
so  that  it  would  run  with  a  dangerous  speed,  or,  as  it  is  called, 
would  "  run  away/'  almost  like  a  shunt  motor  the  shunt  circuit 
of  which  is  disconnected.  Hence  series  motors  are  never  em- 
ployed where  the  load  may  be  entirely  removed.  For  driving  by 
means  of  belts,  for  instance,  series  motors  are  generally  not  em- 
ployed, because  a  sudden  release  of  load  may  cause  the  belt  to  be 
ruptured  or  thrown  off  the  pulley.  On  the  other  hand,  they  are 
more  frequently  used  for  driving  pumps,  fans,  and  so  on,  by  means 


140  ELECTRICAL   ENGINEERING 

of  couplings,  or  for  driving  any  machines  by  gearing.  The  latter 
itself  provides  a  certain  load  on  account  of  its  frictional  resistance  in 
the  toothed  wheels  and  bearings.  Very  small  motors  may,  even  with 
belts,  be  built  as  series  motors,  as  their  comparatively  large  frictional 
resistance  in  the  bearings  represents  in  any  case  a  certain,  although 
small,  load,  allowing  the  motor  to  reach  a  rather  high,  but  not  a 
dangerous  speed. 

Series  motors  are  for  two  reasons  employed  in  some  cases  with 
great  advantage.  A  single  line  only  proceeds  from  the  starter  to  the 
motor,  so  that,  together  with  the  direct  return  wire,  two  mains  only 
are  required,  whereas  with  the  shunt  motor  there  are  two  lines  from 
the  starter  to  the  motor,  which,  with  the  return  wire,  necessitates 
the  use  of  three  mains.  This  offers  an  advantage  and  a  saving  of 
cables  when  the  distance  between  motor  and  starter  is  great.  This 
may,  for  instance,  happen  with  a  motor,  coupled  directly  to  a  fan, 
which  is  fixed  on  the  ceiling  of  a  very  high  room,  and  has  to  be  con- 
trolled by  a  starter,  fixed  below.  Since  the  load  of  such  fan  motors 
is  constant,  the  speed  of  the  series  motor  will  also  remain  con- 
stant. 

As  we  know,  the  magnetic  field  of  a  series  motor  is  stronger  the 
heavier  its  load.  This  makes  it  suitable  for  many  special  appli- 
cations, such  as  lifting  weights  by  means  of  cranes.  A  small 
weight  is  more  quickly  lifted  than  a  heavy  one.  If  a  series  motor 
is  started  under  full  load,  it  wants  less  current  than  a  shunt  motor 
of  the  same  size.  For  let  us  presume  the  magnets  of  a  series  motor 
to  be  wound  so  as  to  produce,  with  a  current  of  25  amps.,  a  mag- 
netic field  equal  in  strength  to  that  of  a  corresponding  shunt  motor. 
If,  further,  we  assume  that  the  motors  have  to  start  under  a  very 
heavy  load,  so  that  the  starting  current  grows  to  50  amps.,  then  it 
is  clear  that  the  field  of  the  series  motor  will  increase  as  well,  although 
not  to  a  double  value.  Naturally  the  armature  of  the  series  motor, 
running  now  in  a  stronger  magnetic  field,  is,  with  twice  the  current, 
capable  of  developing  more  than  double  "  torque."  Thus  the  series 
motor  has  a  greater  starting  power  than  the  shunt  motor,  since  the 
magnetic  field  of  the  latter  remains  constant  at  all  loads,  and  its 
armature  can,  therefore,  with  twice  the  current,  overcome  only  twice 
the  load. 

Hence  the  series  motor  will  be  able  to  overcome  any  given  over- 
load with  a  little  less  consumption  of  current  than  the  shunt  motor, 
but  will  run  a  little  slower  than  the  latter,  and,  on  starting,  the  series 
motor  will,  with  a  given  current,  come  sooner  to  its  full  speed  than 
the  shunt  motor. 

For  electrically  driven  cranes,  as  well  as  for  electric  railways  and 
motor  cars,  series  motors  are  employed  with  great  advantage.  The 
starting  of  an  electric  car  can  be  effected  more  quickly  with  a  series 
than  with  any  other  motor.  On  gradients  the  car  is  running  slower 


THE  ELECTRIC  MOTOR 


141 


Motor 


and  does  not  require  so  much  current  as  one  equipped  with  a  shunt 
motor,  whereas  on  the  level  the  series  motor  enables  the  car  to  run 
with  a  far  higher  speed. 

In  all  our  discussions  we  have  hitherto  assumed  that  the  series 
motor  is  supplied  with  a  constant  voltage.  If  we  want  it  to  run 
with  a  nearly  constant  speed  at  varying  loads,  we  have  to  switch 
the  motor  on  a  low  voltage  if  it  is  loaded  but  little,  and  on  a  higher 
voltage  if  it  is  loaded  to  a  greater  extent.  This  may  be  effected  by 
a  series  resistance,  because  with  a  small  load  we  could  switch  in  much, 
and  at  a  greater  load  less  resistance. 

This  voltage  regulation  may  be  rendered  quite  automatic  by 
employing  a  series  dynamo  as  source  of  current  for  the  motor,  an 

arrangement    which    is       

sometimes  made  for 
power  transmission  to 
long  distances  (see  Fig. 
147).  The  mains  lead 
in  this  case  from  the 
dynamo  to  a  single 
motor  only.  A  starter 
is  not  required  between 
the  t\\o  machines,  but 
the  starting  of  the 
motor  is  done  in  the 
following  way :  The 
dynamo  is  run  by  the 
steam  -  engine  or  the 
turbine  coupled  to  it,  the  motor  being  stationary  and  the  circuit 
closed,  the  resistance  of  the  mains  and  of  the  motor  alone  being  in 
the  external  circuit  of  the  dynamo.  Since  these  resistances  are 
comparatively  small,  even  when  the  dynamo  runs  slowly  a  large 
current  will  flow  through  the  circuit.  This  strong  current  in  the 
armature  and  the  field  of  the  motor  will  cause  it  to  start,  thus  pro- 
ducing a  certain  back  E.M.F.  The  faster  the  dynamo  runs  the  faster 
the  motor  runs,  and  the  speeds  will  always  be  in  the  same  ratio, 
for,  since  the  same  current  is  flowing  through  both  dynamo  and 
motor,  their  magnetic  fields  are  always  of  equal  strength.  Owing 
to  the  loss  of  volts  in  the  mains  and  the  machines  themselves, 
the  back  E.M.F.  of  the  motor  will  always  be  a  little  smaller 
than  the  E.M.F.  of  the  dynamo.  Hence,  if  the  machines  are  ap- 
solutely  alike,  the  motor  will  always  run  a  little  slower  than  the 
dynamo. 

When  the  load  is  small  the  motor  only  takes  a  small  current,  the 
dynamo,  through  the  coils  of  which  this  small  current  is  also  flow- 
ing hence  producing  a  small  E.M.F.  At  an  increased  load  the  cur- 
rent, and  with  it  the  voltage  of  the  dynamo,  increases.  The  speed 


FIG.  147. — Series  Method  of  Power  Transmission. 


142 


ELECTRICAL  ENGINEERING 


of  the  motor  is  but  little  altered  in  this  case,  since  it  remains  in  a 
nearly  constant  ratio  to  the  dynamo  speed.  Thus,  if  the  dynamo  is 
driven  at  a  constant  speed,  that  of  the  motor  will,  even  at  varying 
loads,  remain  practically  constant. 


General  Electric  Co., 
Engineering  Depty, 

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20          6         200 
10          4          100 
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9  may  f904, 

Ra  Iway  motor 
GE-57-A-3               Chan»cter,sticno.2J( 

50  H  P.  output  at  93  Amp.  input. 
Volts  at  motor  terminals  500. 
Diameter  of  car  wheel  33*. 
Armature  3  turns,  Field  Spools  1  10  turns.  , 
Pinion  28,  Gear  57,  Ratio  2.04. 

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FIG.  148.  —Speed  and  Torque  Curves,  Series  Motor. 

The  speed  and  torque  curve  of  a  series  railway  motor  is  shown  in 
Fig.  148,  from  which  can  be  seen  the  variation  of  these  factors  with 
amperes  taken  by  the  motor.  Fig.  148  also  shows  the  speed  curve 
of  a  shunt  motor. 


THE  ELECTRIC  MOTOR 


143 


The  Compound  Motor 


A  compound  winding  may  be  used  on  motors  for  many  different 
purposes.  If  the  current  flows  in  the  same  direction  through  both 
windings,  then  the  effect  of  the  series  coil  strengthens  that  of  the 
shunt  coil.  This  strengthening  is  greater  the  larger  the  armature 
current,  i.e.  the  heavier  the  motor  load.  Thus  the  motor  gets  at 
increasing  load  a  stronger  magnetic  field,  and  will,  therefore,  if  the 
voltage  remains  constant,  run  slower  than  before.  We  hence 
infer  that,  for  a  given  current,  the  starting  power  of  a  compound 


FIG.  149. — Compound  Motor  started  from  a  distant  point. 

motor  will  be  greater  than  that  of  a  shunt  motor.  With  a  decreas- 
ing load  the  motor  will  run  faster.  A  "running  away,"  however, 
cannot  occur,  because,  even  if  the  load  be  taken  off  entirely,  the  shunt 
coil  produces  a  magnetic  field  of  sufficient  strength.  The  compound 
motor  has,  therefore,  to  a  certain  extent,  the  merits  of  the  series 
motor  without  its  disadvantages. 

By  means  of  compound  motors  the  starting  at  a  distance  with  only 
two  mains  may  be  effected,  just  as  in  the  case  of  the  series  motor. 
In  Fig.  149  a  diagram  for  such  a  connection  is  shown.  If  we  imagine 


144  ELECTRICAL  ENGINEERING 

the  motor  without  the  shunt  coil,  then  it  is  connected  up  exactly  as 
the  series  motor  in  Fig.  146.  The  current  coming  from  the  starter 
enters  the  series  coil  in  VI.,  flows  through  the  series  coil  and  leaves 
it  at  V.,  flowing  from  there  to  the  armature  brush  II.,  through  the 
armature  to  brush  I.,  and  from  there  through  the  second  main  back 
to  the  generator.  The  shunt  winding  is  connected  directly  with  the 
armature  brushes  I.  and  II.,  and  gets  at  starting,  therefore,  a  very 
small  voltage  only,  hence  its  field  is  nearly  ineffective.  But  on  account 
of  its  series  winding,  the  motor  starts  as  a  series  motor.  Obviously 
such  a  motor  will  not  develop  a  very  large  starting  power,  like  a  real 
series  motor,  for,  on  account  of  the  large  space  occupied  by  the  shunt 
coils,  there  is  less  space  available  for  the  series  coils  than  with  a  series 
motor.  A  compound  motor  may,  however,  even  with  this  arrange- 
ment, be  easily  got  into  motion,  provided  that  the  load  on  starting  is 
not  too  heavy.  When  once  running  the  armature  will  produce  a 
back  E.M.F.,  and  the  shunt  coil  will  be  supplied  with  nearly  the  full 
terminal  voltage. 

This  arrangement  for  starting  at  a  distance  may  be  employed  in 
cases  in  which  the  motor  is  not  coupled  directly  to  a  pump,  fan, 
etc.,  but  is  driving  the  latter  by  means  of  a  belt.  Even  if  the  belt 
slips  off  the  motor  the  latter  cannot  run  away,  as  would  be  the  case 
with  a  series  motor. 

Sometimes  it  is  wished  to  produce  another  effect  with  the  com- 
pound winding.  As  we  know,  the  speed  of  a  shunt  motor  does 
not  remain  absolutely  constant  at  all  loads.  Generally  it  decreases 
a  little  at  an  increasing  load.  Now  there  are  some  cases  in 
which  an  absolutely  constant  speed  is  required,  such  as,  for  instance, 
when  driving  spinning  machines.  This  may  be  got  by  winding 
over  the  shunt  coil  a  series  coil,  consisting  of  a  few  windings  only, 
and  which  act  in  an  opposite  direction.  The  result  is  that,  as  the  load 
becomes  heavier,  the  field  of  the  motor  is  weakened,  and  the  armature 
runs  faster.  Since  now,  on  the  other  hand,  the  motor  would  run  slower 
at  an  increasing  load  if  it  were  a  shunt  motor  only,  this  fall  of  the 
speed  is  compensated  by  the  action  of  the  series  winding.  Thus  a 
compound  winding  is  capable  of  giving  a  constant  speed  at  all  loads. 

This  statement  is  not  absolutely  true.  There  is  a  further 
reason  for  the  variation  of  speed,  which  cannot  be  compensated 
by  the  series  coil,  namely,  the  gradually  rising  temperature  of  the 
motor.  The  resistance  of  the  shunt  coils  is  greater  when  hot  than 
when  cold,  and  if  the  coils  are  switched  on  to  a  constant  voltage, 
a  larger  current  will  flow  through  them  if  they  are  cold  than  if 
they  are  hot.  After  the  motor  has  been  running  for  some  time, 
its  magnetism  will  gradally  become  a  little  weaker.  We  may 
therefore  observe,  with  shunt  motors,  that  the  speed  of  the  motor  is 
smaller  immediately  after  starting,  but  grows  gradually  with  the  rise 
of  temperature.  This  increase  of  speed  lasts  only  for  a  short  time. 


THE  ELECTRIC   MOTOR 


145 


After  some  hours  running,  the  motor  does  not  get  hotter,  since  it 
gives  the  heat  produced  in  it  to  the  surrounding  air.  After  the 
motor  has  reached  this  state,  its  speed  remains  constant,  providing 
that  there  has  been  no  change  in  the  voltage. 

This  influence  of  the  temperature  may  be  done  away  with,  by 
connecting  up  in  the  shunt  circuit  of  the  motor  a  small  regulating 
resistance,  as  shown  in  Fig.  145.  Before  starting,  when  the  resistance 
of  the  coils  is  lower,  some  resistance  is  switched  in  the  shunt  circuit, 
and,  as  the  coils  heat  up  and  increase  in  resistance,  the  auxiliary 
resistance  is  gradually  short-circuited. 

It  must  be  added  that  compound  windings  are  not  much  used  for 
running  motors  at  constant  speed. 


Direction  of  Rotation  of  a  Motor 


To  alter  the  direction  of  rotation  of  a  motor  we  have  either  to 
change  the  direction  of  the  armature  current,  or  to  reverse  the 

polarity  of  the  magnetic  field.  If 
we  reverse  the  armature  current 
and  the  polarity  of  the  magnetic 
field  simultaneously,  the  direction 
of  rotation  will  naturally  remain 
the  same  as  before. 

Fig.  150  shows  the  diagram  of 
connections  for  a  series  motor, 
which,  seen  from  a  certain  side, 
rotates  counter-clockwise.  The 
current  is  flowing  in  the  magnet 
coils  from  terminal  VI.  to  terminal 
V.,  and  in  the  armature  from 
brush  II.  to  brush  I.  For  re- 
versing the  direction  of  rotation, 
we  may  either  leave  the  direction 
of  the  magnet  current,  and  alter 

that  of  the  armature  current  by  changing  the  two  cables  leading  to 
the  brushes,  thus  connecting  brush  I.  with  magnet  terminal  V.,  and 
brush  II.  with  the  second  main,  as  shown  in  Fig.  151;  or  we 
may,  as  shown  in  Fig.  152,  leave  the  direction  of  the  armature 
current,  and  reverse  that  of  the  magnet  current. 

There  would  be  no  reversal  of  the  motor  if  we  changed  the  mains 
leading  to  the  starter  and  to  the  motor  directly,  since  in  this  case 
both  the  armature  and  the  magnet  current  would  be  reversed. 

Similar  diagrams  of  connections  for  the  shunt  motor  are  shown 


FIG.  150. — Series  Motor — Counter- 
clockwise rotation. 


146 


ELECTRICAL  ENGINEERING 


in  Figs.  153-155.  In  Fig.  153  the  armature  is  rotating  counter- 
clockwise. The  armature  current  is  flowing  from  brush  II.  to  brush 
I.,  the  magnet  cunen*  from  terminal  IV.  to  III.  Fig.  154  shows 


FIG.  151. — Series  Motor — Clockwise 
rotation. 


FIG.  152. — Series  Motor — Clockwise 
rotation. 


how  the  armature  current  may  be  reversed,  whilst  the  magnet  current 
remains  in  the  same  direction,  and  Fig.  155  how  the  magnet  current 
may  be  reversed  without  changing  the  armature  current. 

Great    care    must    be    taken    to    always    connect    the    magnet 


FIG. 


153.— Shunt  Motor— Counter- 
clockwise rotation. 


FIG.  154. — Shunt  Motor — Clockwise 
rotation. 


terminals  so  as  to  get  the  full  terminal  voltage  on  them  as  soon 
as  the  lever  touches  the  first  contact  piece.  This  full  terminal 
voltage  has  to  remain  on  the  magnets  during  the  whole  starting 


THE  ELECTRIC  MOTOR 


147 


period,  and  also  when  the  starter  has  been  short-circuited.  If  this 
is  not  the  case,  the  consequences  may  be  serious.  If,  in  changing  the 
armature  cables  as  per  Fig.  154,  we  had  not  connected  magnet 
terminal  III.  with  the  main,  but  had  left  it  on  brush  I.  (see 
Fig.  156),  then,  at  starting  the  motor  the  following  would  take  place. 
If  we  put  the  lever  on  the  first  contact,  the  current  will  flow  through 
the  whole  of  the  resistance,  the  latter  consuming  the  greatest  part  of 
the  voltage.  Magnet  terminal  IV.  is  connected  by  means  of  the 
slip-ring  directly  with  the  main  leading  to  the  starter,  whereas 
terminal  III.  is  connected,  not  with  the  return  main,  but  with  brush  I., 
a  cable  leading  from  this  brush  to  the  last  contact  piece  of  the 
starter.  As  long  as  the  motor  is  stopped,  there  is  only  a  very 
small  voltage  between  I.  and  the  return  main  II.,  the  magnets  are 


FIG.  155. — Shunt  Motor — Clockwise 
rotation. 


FIG.    156.— Shunt  Motor  with 
Wrong  Connection. 


on  nearly  the  full  voltage,  the  magnetic  field  will  therefore  have 
nearly  its  full  strength,  and  the  motor  will  start  to  run.  If,  then, 
the  motor  is  running,  it  will  produce  a  back  E.M.F.,  and  this 
voltage,  arising  between  I.  and  II.,  will  gradually  diminish  the  voltage 
between  I.  and  the  main  leading  to  the  starter.  But  on  this  latter 
voltage  the  magnets  are  connected.  Thus  the  magnetic  field  will 
become  weaker  in  the  same  proportion  as  the  motor  runs  faster. 
If  finally  we,  as  is  generally  done,  short-circuit  the  starter,  then 
the  voltage  between  the  two  magnet  terminals  becomes  nil,  there 
would  be  practically  no  magnetic  field,  hence  the  motor  would  either 
"run  away,"  or  the  fuses  would  go.  It  would  also  be  wrong,  to 
connect  the  two  magnet  terminals  directly  with  the  two  armature 
brushes,  or,  what  would  be  the  same  thing,  to  connect  magnet 
terminal  III.  with  the  armature  brush  I.,  and  magnet  terminal  IV. 


148 


ELECTRICAL  ENGINEERING 


with  the  short-circuiting  contact  of  the  starter,  instead  of  connecting 
it  with  the  slip-ring  (see  Fig. 
157).  In  this  case  the  magnets 
would  at  starting  not  get  the  full 
voltage,  but  only  that  of  the 
armature;  and  since,  due  to  the 
starting  resistance,  the  latter  is 
very  small  at  the  start,  the 
magnetic  field  would  be  a  very 
small  one  too.  Thus  the  motor 
can,  if  it  is  not  loaded,  start, 
but  will  consume  a  very  large 
current  in  doing  so.  If,  how- 
ever, the  motor  is  loaded,  it  will, 
owing  to  the  weak  magnetic  field, 
not  be  able  to  start  at  all.  If, 
on  the  other  hand,  the  motor  is 
running,  producing  hereby  a  back 


FIG.  157 — Shunt  Motor  with  Wrong 
Connection. 


E.M.F.,  the  voltage  of  the  magnet 

winding  will  gradually  grow.  When  at  last  the  starting  resistance 
is  short-circuited,  the  magnet  will  be  excited  with  full  terminal 
voltage.  Thus  a  wrong  connection,  as  described  here,  makes  starting 
impossible,  or  renders  it  at  least  very  difficult,  but  if  once  started 
the  motor  will  run  all  right.  The  wrong  connection  described  be- 
fore, allows  proper  starting,  but  renders  working  of  the  motor  im- 
possible. 

For  all  cases  the  following  rule  for  connecting  up  a  shunt  motor 
should  be  noted  by  the  student:  One  pole  of  the  mains  to  be  con- 
nected to  a  terminal  common  to  the  armature  and  the  field;  the  second 
pole  of  the  mains  to  be  led  to  the  starter,  and  to  be  branched  there 
in  such  a  way  as  to  get  at  starting  the  full  voltage  on  the  second 
magnet  terminal,  while  there  is  still  in  use  the  whole  starting  resist- 
ance in  the  armature  circuit.  The  latter  is  then  gradually  to  be  short- 
circuited  during  starting. 

For  reversing  the  direction  of  rotation  of  a  compound  motor  we 
have  either  to  reverse  the  armature  current  or  that  of  the  shunt  and 
series  coils  simultaneously.  If  we  changed  the  connections  of  the 
shunt  coil  only,  the  motor  would  work  quite  differently.  Consider, 
for  instance,  those  connections  with  which  we  have  become  acquainted 
for  starting  at  a  distance  (see  Fig.  149):  the  following  would  happen: 
In  the  beginning,  when  the  series  coil  only  acts,  the  motor  would 
start  to  run  in  a  certain  direction,  but  then  the  shunt  coil,  acting 
oppositely,  weakens  the  field  so  much  as  to  cause  the  motor  to  run 
away. 

The  reversal  of  direction  of  rotation  may  with  many  motors, 
especially  with  multipolar  ones,  be  done  simply  by  moving  the 


f 

THE  ELECTRIC  MOTOR  149 

brushes  to  another  position,  so  that  they  are  shifted  the  width 
of  a  pole  from  their  former  position.  This  causes  the  direction 
of  the  armature  current  to  be  reversed,  and  thus  nothing  further  is 
needed. 


Armature  Reaction  with  Motors 


With  motors  there  is  an  armature  reaction  of  the  same  kind  as 
with  dynamos,  causing  a  weakening  of  the  magnetic  field.  The 
armature  reaction  is  greater  the  stronger  the  current  flowing  through 
the  armature.  Thus,  with  shunt  motors  under  load,  the  field  will 
be  somewhat  weaker  than  at  no  load.  The  motor  will,  therefore, 
due  to  the  armature  reaction,  run  somewhat  faster  under  the 
bigger  load  if  it  were  not  for  an  Ohmic  voltage-drop  in  the  armature. 
Since  this  voltage  loss  tends  to  decrease  the  speed  with  increased 
load,  there  is  generally  no  action  of  the  weakened  field  to  be  observed; 
on  the  contrary,  there  generally  occurs  on  loading  the  motor  a  de- 
crease of  its  speed. 

With  motors  having  considerable  armature  reaction,  it  may 
happen  that  the  speed  increases  with  the  load;  but  in  many  cases 
the  action  of  the  armature  reaction  and  that  of  the  Ohmic  voltage- 
drop  compensate  each  other,  so  that  the  motor  speed  remains  prac- 
tically constant. 

With  series  motors  the  armature  reaction  is  of  less  consequence 
because  the  main  field  is  strengthened  on  increasing  the  load. 

With  motors  which  do  not  run  without  sparking  at  various  loads 
an  adjustment  of  brushes  is  required  as  the  load  varies.  This  move- 
ment of  the  brushes,  the  student  should  remember,  has  at  an  in- 
creasing load  not  to  take  place  in  the  direction  of  rotation  as  with 
dynamos,  but  in  an  opposite  direction.  To  reverse  the  current  in 
the  armature  coil  that  happens  to  be  short-circuited  by  the  brush, 
we  have  to  bring  the  latter  within  reach  of  a  weak  magnetic  field, 
which  induces  an  E.M.F.  opposite  to  that  which  was  previously  in- 
duced in  the  coil.  But,  as  we  are  aware,  in  each  winding  of  the  motor 
armature  under  the  influence  of  a  pole  an  E.M.F.  is  induced,  which 
tends  to  produce  a  current  in  an  opposite  'direction.  Thus  we  have 
only  to  short-circuit  each  winding  before  it  comes  beyond  the  in- 
fluence of  the  magnet  pole.  We  therefore  have  to  displace  the  brushes 
from  the  middle  of  the  neutral  zone  backwards,  and  not  forwards, 
as  with  a  dynamo  (see  Fig.  158). 

In  comparing  Fig.  158  with  Fig.  127,  we  see  that  the  direction  of 
the  current  and  displacement  of  brushes  are  the  same  as  before;  but 
the  direction  of  rotation  of  the  motor  has  been  changed.  We  thence 


150 


ELECTRICAL  ENGINEERING 


note  that  the  displacement  of  brushes  has  to  be  done  in  the  direction 
of  rotation  with  a  dynamo,  but  opposite  to  the  direction  of  rotation  with 
a  motor. 

With  motors  which  have  to  run  in  both  directions  (reversible 


FIG.  158. 

motors)  it  is  naturally  impossible  to  displace  the  brush-rocker  at  each 
change  of  the  direction  of  rotation.  These  motors  have  to  be  designed 
so  as  to  run  without  sparking,  and  without  any  displacement  of  the 
brushes  whatever  being  necessary. 


Reversing  Apparatus 

In  many  cases — such  as,  for  instance,  with  lifts,  cranes,  electric 
trams,  and  so  on — it  is  necessary  to  have  the  motors  running  at  first 
in  one  and  then  in  the  other  direction.  In  such  cases  it  is,  of  course, 
impracticable  to  continually  alter  the  position  of  the  cables  or  the 
brushes. 

Quick  reversal  may  be  effected  by  means  of  a  "double-pole, 
throw-over"  switch.  This  switch,  the  diagram  of  which  is  shown  in 
Fig.  161,  and  a  general  view  in  Fig.  160,  consists  of  two  levers 
coupled  to  each  other.  The  pivots  of  the  levers  form  electric 
contact-pieces;  the  levers  themselves  are  made  of  metal,  being  insu- 
lated from  each  other.  By  lifting  the  levers  upwards,  contact  a 
is  connected  with  c,  and  b  with  d.  On  pushing  them  do WTI wards, 
contact  a  is  connected  with  e,  and  b  with  /.  As  shown  in  Fig.  159, 
the  contacts  c  and  /,  d  and  e  are  connected  crosswise  with  each  other. 


THE  ELECTRIC  MOTOR 


151 


Contact  d  is  in  connection  with  the  positive,  contact  c  with  the 
negative  main,  whereas  the  middle  contacts  a  and  b  are  in  connection 


FIG.  159. — Shunt  Motor  with  Change- 
over Switch. 


FIG.  160.— Two-Pole 
Change-over  Switch. 


od 


a 


with  the  magnet  terminals  of  the  shunt  motor.  On  putting  the  lever 
upwards  we  connect  magnet  terminal  IV.  with  the  positive  pole,  and 

terminal  III.  with  the  negative  pole  of  the 
mains,  hence  the  current  is  flowing  in  the 
magnet-coil  in  the  direction  from  IV.  to 
III.  On  putting  the  levers  downwards 
we  connect  terminal  IV.  with  the  nega- 
tive pole,  and  terminal  III.  with  the  posi- 
tive pole,  of  the  mains,  thus  reversing  the 
direction  of  current  flowing  in  the  coil, 
and,  as  the  current  in  the  armature 
always  keeps  the  same  direction,  we  there- 
fore reverse  the  direction  of  rotation  of 
the  motor. 

Obviously  we  could  also  arrange  the 
throw-over  switch  in  the  armature  circuit 
instead  of  in  the  magnet  circuit. 

Such  a  reversing  device  would,  of 
course,  be  suitable  for  the  purpose,  but 

it  would  be  a  dangerous  one,  for  if  we  reversed  the  switch  whilst  the 
starter  is  short-circuited,  the  sudden  reversing  of  the  motor  might 
cause  its  destruction. 


e 


O         Of 

FIG.  161.— Two-Pole 
Change-over  Switch, 


152 


ELECTRICAL  ENGINEERING 


To  prevent  accidents  of  this  nature,  the  reversing  switch  is 
generally  rigidly  connected  with  the  starter,  so  as  to  render  the 
reversal  only  possible  when  the  armature  circuit  is  opened.  Such  an 
apparatus  is  called  a  reversing  and  starting  switch.  The  diagram  of 
connections  for  this  apparatus  is  shown  in  Fig.  162.  The  left  and 
right  half  of  the  apparatus  are  quite  symmetrical.  The  single  resist- 
ance spirals  (marked  by  the  vertically  drawn  zigzag  line)  are  con- 
nected both  with  the  contacts  1,  2,  3,  .  .  .  9  to  the  left,  and  with  the 
contacts  1,  2,  3,  ...  9  to  the  right.  Those  marked  1  represent 


FIG.  162. — Starting  and  Reversing  Switch  for  Shunt  Motor. 


the  short-circuiting  contacts.  There  are  further  some  circularly 
arranged  half-rings ;  the  small  ones  that  are  innermost  are  connected 
with  the  magnet  terminals  III.  and  IV.,  whereas  the  next  wider  ones  are 
connected  with  the  positive  and  negative  main  respectively.  Armature 
brush  II.  is  connected  with  the  large  outermost  half  slip-ring,  whereas 
armature  brush  I.  ip  in  connection  with  short-circuiting  contact  1. 
On  either  side  of  the  starting  lever  there  are  fixed  brushes  Bt  and 
B2,  which  are  insulated  from  each  other,  but  each  of  which  covers 
simultaneously  the  three  circles  on  either  side.  If  now  the  lever  be 


THE  ELECTRIC  MOTOR  153 

put  in  the  middle  (vertically)  neither  of  the  two  brushes  will  cover 
any  of  the  two  current-leading  rings  (marked  as  +  and  — )  because 
these  rings  do  not  extend  so  far.  By  moving,  however,  the  upper 
part  of  the  lever  to  the  left  into  the  position  which  is  shown  in 
Fig.  162,  the  innermost  half  slip-ring  and  the  starting  contact  9  are 
connected  with  the  positive  slip-ring.  Thus  the  current  will  branch, 
flowing  on  one  hand  directly  to  the  magnet  terminal  III.,  on  the  other 
hand  to  the  contact  piece  9,  and  from  there  through  the  whole 
resistance  to  contact  1.  which  is  connected  with  armature  terminal  I. 
At  the  same  time  both  the  magnet  terminal  IV.  and  the  second 
armature  terminal  II.  are  connected  by  means  of  the  lever  brush  B2 
with  the  negative  main,  and  thus  the  motor  can  start  to  run.  It  may, 
for  instance,  run  to  the  left.  If  then  we  move  the  lever  further  to 
the  left,  we  gradually  short-circuit  the  resistance,  till  finally  we 
come  to  contact  1 ,  when  the  armature  is  connected  directly  with  the 
positive  main,  and  the  motor  running  with  its  full  speed. 

If,  however,  we  move  the  lever  from  its  middle  position  towards 
the  right,  instead  of  moving  it  to  the  left  as  before,  the  brush  B1? 
covering  the  slip-ring  marked  - ,  connects  the  negative  main  both 
with  the  magnet  terminal  IV.  and,  through  the  resistance,  with 
armature  terminal  I.  At  the  same  time  the  lower  brush,  B2,  connects 
the  positive  main  with  the  magnet  terminal  III.  and  the  armature 
terminal  II.  Thus  the  current  is  flowing  through  the  shunt  coil  in  the 
same  direction  as  before,  but  in  an  opposite  direction  through  the 
armature.  The  motor  will  therefore  run  in  the  opposite  direction. 

To  prevent  the  lever  from  being  turned  more  than  a  quarter  turn 
on  either  side,  there  are  arranged  two  stops,  a,  on  the  apparatus. 

Other  reversing  and  starting  switches  are  designed  so  as  to  reverse 
the  magnet  current,  whilst  the  armature  current  remains  in  the  same 
direction. 

Starting  and  reversing  switches  for  series  motors  are  constructed 
in  a  very  similar  manner. 

In  Fig.  163  the  construction  of  a  simple  reversing  and  starting 
switch  is  shown. 


Sparking  with   Starters    and    Shunt   Regulators 

When  a  shunt  circuit  is  broken  a  much  longer  spark  results  than 
in  the  case  of  a  lamp  circuit  of  equal  current  strength  and  voltage. 
The  reason  of  this  strong  sparking  lies  in  a  property  of  the  electric 
current,  which  is  called  self-induction,  and  with  which  we  shall  deal 
later  on,  in  a  more  detailed  fashion. 


154 


ELECTRICAL  ENGINEERING 


In  a  winding  surrounding  an  iron  core,  an  E.M.F.  is  induced  as 
soon  as  we  alter  the  strength  of  magnetism  of  the  iron  core  (see  p.  67). 
If,  now,  the  strength  of  magnetism  is  changed  by  altering  the  current 
flowing  round  the  core,  there  will  be  produced  an  induction  effect  in 
the  coil  resulting  in  a  certain  E.M.F.  of  "  self-induction. " 

If  a   rapid   alteration   of  the   current   occurs — for  instance,   on 
breaking  a  circuit  very  quickly — then  at 
this  moment  a  far  greater  E.M.F.  may  be 
induced  than  existed  before. 

The  E.M.F.  of  the  self-induction  resists 
any  alteration  of  the  current,  it  tends  to 
maintain  the  current  at  its  original  strength, 
just  as  the  inertia  does  not  allow  a  moving 
body  to  stop  immediately  the  driving  force 
ceases.  If  a  running  vehicle  is  suddenly 
stopped  in  its  course  by  any  impediment, 
such  as  a  wall  or  a  door,  then  the  sudden 
stop  will  cause  a  force  sufficient  to  destroy 
the  wall  or  door.  Here  a  far  greater  force 
is  produced  than  had  to  be  spent  previously 
in  continuously  moving  the  vehicle. 

It  is  exactly  the  same  on  stopping  an 
electric  current.  The  large  E.M.F.  of  self- 
induction  produced  on  the  sudden  discon- 
nection of  a  110  volt  shunt  circuit  sometimes  destroys  the  insula- 
tion of  coils  which  could  have  withstood  a  voltage  of  even  500, 
and  might  start  an  arc  which  the  normal  voltage  would  be  un- 
able to  keep  up.  As  a  consequence, 
the  ends  of  the  shunt  slip-rings  and 
the  corresponding  contact  brushes 
of  starters  are  generally  burnt  out 
after  a  short  time. 

To  avoid  this  we  must  adhere 
to  the  rule  of  never  breaking  a 
shunt  circuit.  Referring  to  our 
analogy,  the  vehicle  must  not  be 
stopped  suddenly,  but  allowed  to 
come  to  rest  gradually.  This  may, 
in  .our  case,  be  effected  by  making 
the  connections  between  motor  and 
starter  according  to  the  diagram  in 
Fig.  164.  By  this  arrangement  it 
is  possible  to  switch  the  motor  off 
the  main  without  disconnecting  the 
shunt  circuit.  As  may  be  seen  from  the  diagram,  the  shunt  slip-ring 
is  in  connection  with  the  first  resistance  contact.  Starting  the  motor 


FIG.  163.— Motor  Starting 
Switch  ( t  ereingt e  E.  A. 
G.,  Vienna). 


FIG.  164. — Starter  with  Inductionless 
Break,  having  Shunt  Slip-Ring. 


THE  ELECTRIC  MOTOR  155 

has  to  be  done  as  with  the  usual  starter.  When  the  lever  is  put  from 
the  dead  contact  6  to  the  first  resistance  contact  5,  the  shunt  coils  get 
full  voltage,  for  the  slip-ring  is  connected  with  this  contact.  The 
armature,  as  usual,  is  switched  in  series  with  the  whole  resistance. 
If,  then,  we  move  the  lever  gradually  to  the  left — for  instance,  to 
contact  3 — the  shunt  coils  remain  connected  with  the  full  voltage, 
because  the  lever  always  touches  the  slip-ring.  The  armature,  how- 
ever, is  no  longer  in  series  with  the  whole  of  the  resistance,  but  only 
with  the  part,  which  is  between  contact  3  and  1.  The  resistance 
spirals  between  5  and  3  are  without  current.  Contact  5.  of  course, 
is,  by  means  of  the  shunt  slip-ring,  in  connection  with  the  starting 
lever,  and  thus  with  one  main;  but  contact  3  is  also  in  connection 
with  the  lever  and  the  main;  hence  this  part  of  the  resistance  (viz. 
that  between  5  and  3)  is  connected  at  both  ends  with  one  pole  only. 
Between  the  ends  of  this  part  of  the  resistance  there  is  no  voltage, 
and  thus  no  current  can  flow  through  it.  It  will  be  exactly  the  same 
if  we  gradually  short-circuit  the  motor.  Thus  we  see  that  there  is  no 
difference  whatever  in  starting  by  means  of  this  apparatus  compared 
with  starting  by  means  of  the  usual  apparatus.  In  starting,  the  motor 
produces,  as  we  know,  a  back  E.M.F.,  which  is  nearly  equal  to  the 
voltage  of  the  current.  If  now  we  switch  out  the  motor  quickly,  we 
do  not  interrupt  the  armature  and  the  magnet  circuit  as  we  did  with 
the  usual  apparatus.  We  break,  of  course,  the  outer  circuit,  but 
there  is  another  closed  circuit  in  the  motor  itself,  viz.  that  from 
armature  brush  II.  through  the  whole  resistance,  from  there  over  the 
shunt  slip-ring  to  magnet  terminal  IV.,  through  the  magnet  coil,  and 
from  magnet  terminal  III.  back  to  armature  brush  I.  Now  the 
armature  has  at  the  moment  of  the  break,  if  this  occurs  quickly  enough, 
still  its  full  speed,  and  thus  its  full  back  E.M.F.  This  latter  produces, 
if  there  is  a  closed  circuit,  a  current  opposite  to  the  previous  one. 
Thus  this  current  leaves  brush  II.,  flowing  through  the  resistance  from 
1  to  5,  the  magnet  coils  from  IV.  to  III.,  and  enters  the  armature 
again  by  brush  I.  The  current  flows  through  the  magnets  in  the 
same  direction  as  before.  As  no  interruption,  and  not  even  a  sudden 
alteration  of  the  magnet  current  has  taken  place,  there  cannot  be 
produced  a  considerable  E.M.F.  of  self-induction,  and  thus  there  will 
be  no  sparking. 

This  starter,  with  "  self  -indue  tionless  break,"  has  been  further 
simplified  by  omitting  the  shunt  slip-ring,  and  connecting  the 
magnet  terminal  IV.  directly  with  the  first  resistance  contact  5 
(see  Fig.  165).  There  is  obviously  no  alteration  with  regard 
to  the  self-inductionless  break  when  compared  with  the  previous 
case.  On  starting,  however,  there  is  an  alteration.  In  putting 
the  lever  on  contact  5,  the  shunt  coil  gets  full  voltage  as  before. 
But  if  we  now  bring  the  lever,  for  instance,  to  contact  3,  magnet 


156 


ELECTRICAL  ENGINEERING 


FIG.  165.— Starter  with  Indue tionless 
Break,  without  Slip-Ring. 


terminal  IV.  is  no  longer  connected  directly  with  the  starting 
lever,  but  is  in  series  with  the  resistance  between  the  contacts 
3  and  5.  In  putting  the  lever 
on  the  short-circuiting  contact 
1,  the  magnet  coil  will  be  in  f> 
series  with  the  whole  starting 
resistance,  thus  the  magnet  cur- 
rent will  be  weakened.  This  is 
of  little  importance,  for,  since  the 
resistance  of  the  starting  spirals 
is  very  small,  the  voltage  con- 
sumed by  the  spirals,  and  thus  the 
weakening  of  the  magnet  current, 
will  be  negligible.  Suppose,  for 
instance,  that  the  resistance  of 
the  starting  coils  is  5&>,  so  that 
the  armature  current,  with  the 
lever  on  the  first  contact,  is 
with  110  volts  about  20  amps., 
the  normal  shunt  current  being  2  amps.,  and  thus  the  shunt 
resistance  55&>.  In  the  diagram,  Fig.  165,  we  have  then,  with  a 
short-circuited  starter,  a  shunt  resistance  of  5  +  55  =  60&>,  and  thus 
a  shunt  current  of  -y/  =  1.83  amps.,  against  the  2  amps,  previously. 
This  small  weakening  of  the  magnetic  field  will  cause  the  motor 
to  run  a  little  faster. 

A  sparkless  breaking  of  the  motor  circuit  can  only  be  effected 
if  there  is  at  the  moment  of  switching  out  no;  or  a  very  small, 
pressure  difference  between  the  starting  lever  and  the  last  contact. 
Thus,  to  get  a  sparkless  breaking  of  the  motor  circuit  with  a  starter 
such  as  Fig.  164  or  165,  a  rapid  switching  out  is  required.  For, 
if  we  moved  the  lever  slowly  from  one  contact  to  another  one,  the 
speed,  and  with  that  the  back  E.M.F.,  would  gradually  decrease,  so 
that  finally,  if  the  back  E.M.F.  be  only  a  very  small  one,  we  have  to 
break  a  large  current  at  the  full  voltage,  thus  getting  a  long  spark  in 
spite  of  the  " self-indue tionl ess"  connection. 

Sometimes  it  is  impossible  to  avoid  the  interruption  of  the 
shunt  circuit.  Here  we  are  generally  helped  by  closing  the  shunt- 
circuit  on,  itself  whilst  it  is  still  being  switched  out,  so  that  the 
self-induction  current  may  flow  in  the  circuit  so  formed.  For  a 
dynamo,  this  is  shown  in  Fig.  166.  With  the  exception  of  the 
dead  contact  the  arrangement  of  the  shunt-regulator  is  quite  a 
normal  one.  The  dead  contact,  however,  which  is  usually  without 
any  connection  whatever,  is  now  connected  with  the  shunt  terminal 
III.,  and,  since  the  latter  is  connected  directly  with  the  armature 
brush  I.,  also  with  this.  Hence,  if  we  come  from  the  last  resistance 


THE  ELECTRIC   MOTOR 


157 


contact  to  the  dead  one,  the  shunt  is  short-circuited  on  itself,  and 

the  self-induction  current  pro- 
duced on  breaking  flows  in  thi< 
circuit.  Since  the  lever  covers  for 
a  moment  both  the  last  resistance 
and  the  dead  contact,  we  get. 
during  this  time,  a  current 
from  armature  brush  II.  through 
the  resistance  spirals  and  the 
connecting  wire  to  I.,  but  that 
is  no  disadvantage. 

The  switching  out  of  shunt 
regulators  must  not  be  done 
suddenly  like  the  switching  out 
of  starters.  It  is,  on  the  con- 


Fio.  166.— Shunt-Regulator,  with  Con-     fraT_      advisable    to    IPSVP    trip 
nection  for  short-circuiting  the  Mag-     ,trary'    aavlsar> 
net  Coils  in  the  "off"  nosition.  lever  for  some  time  on  the  last 

resistance  contact,  in  order  that 


net  Coils  in  the  "off"  position. 


PIG.  167. — Starting  and  Reversing  Switch  with  Connections  for  short- 
circuiting  the  Magnet  Coils  in  the  "off"  position. 


158  ELECTRICAL  ENGINEERING 

the  voltage  of  the  machine  may  meanwhile  decrease.  If  the 
resistance ^  of  the  shunt  regulator  be  large  enough,  the  machine 
will  lose  its  voltage  almost  entirely.  In  such  a  case,  even  with- 
out the  special  connection  between  the  dead  contact  and  the  second 
magnet  terminal,  an  injurious  self-induction  voltage  and  flashing 
would  not  result. 

A  reversing  and  starting  apparatus  with  "self-inductionless" 
break  is  shown  in  Fig.  167.  If,  in  switching  out,  the  brush  Bt 
leaves  the  wide  slip-ring  and  the  contact  9,  the  armature  and  the 
magnets  are  still  in  connection.  If,  then,  we  place  the  lever  in 
the  middle,  the  two  shunt  slip-rings  are  short-circuited.  Care  must 
be  taken  when  using  this  apparatus,  not  to  move  the  lever  too  soon 
over  the  middle  position,  for  in  this  case  the  circuit  of  the  short- 
circuited  magnet  coils  would  be  again  interrupted  before  the  self- 
induction  current  had  ceased,  and  consequently  the  self-induction 
vvould  cause  considerable  flashing. 

Another  important  mode  of  control  used  for  shunt  or  series  motors 
is  called  the  Ward-Leonard  system  of  control.  In  this  system  the 
motor  field  may  be  separately  excited.  The  armature  of  the  motor 
is  connected  directly  to  the  armature  of  the  generator  without  resist- 
ance. If  there  is  no  field  on  the  generator,  no  E.M.F.  will  be  gen- 
erated, and  no  current  will  flow  to  the  motor.  If  now  a  little  field 
be  put  upon  the  generator,  a  small  E.M.F.  be  generated  in  the  gen- 
erator, a  current  will  flow  at  a  few  volts  to  the  motor,  and  it  will 
slowly  start.  As  the  field  of  the  generator  is  strengthened  the  voltage 
continues  to  increase,  and  the  motor  continues  to  speed  up  until  full 
field  and  full  voltage  is  being  produced  by  the  generator.  Reduction 
of  speed  can  be  effected  by  a  reduction  of  field  of  the  generator. 
Thus,  by  manipulating  the  small  field  current  of  the  generator,  a 
large  armature  current  to  the  motor  can  be  controlled.  The  con- 
troller, therefore,  since  it  handles  such  small  currents,  as  compared 
with  the  currents  doing  the  work,  is  very  many  times  smaller  than 
if  it  were  located  in  the  armature  circuit.  This  method  of  control 
has  a  very  wide  application.  It  is  used  on  battleships,  hoists,  and  to 
control  at  a  distance.  A  good  generator  will  operate  without  sparking 
under  these  low-voltage  high-current  conditions,  for  the  voltage, 
being  low  with  consequently  low  volts  per  bar  on  commutator,  gives 
a  very  favorable  sparking  condition.  As  a  matter  of  fact,  a  good 
generator  will  take  25  per  cent,  over  normal  current  down  to  0  volt 
between  brushes  without  trouble  from  sparking. 


THE  ELECTRIC  MOTOR 


159 


Motors  for  Certain  Purposes 

A  dynamo  can  usually,  without  any  alteration,  be  also  used  as 
a  motor,  but,  since  motors  are  employed  for  so  many  different 


FIG   168.  -Enclosed  Motor. 


FIG.   169.  -Enclosed  Motor     (Electromotors  Company,  Manchester}. 


160 


ELECTRICAL  ENGINEERING 


purposes  for  which  a  special  shape  is  desirable,  a  number  of  types 
of  motors  have  been  designed. 

A  special  form  is  the  enclosed  motor,  which  is  employed  for 
damp  and  dusty  rooms.  The  motor  is  entirely  enclosed  in  a  cast- 
iron  or  steel  case,  which  has  doors  near  the  commutator,  through 
which  the  latter  may  be  inspected  or  cleaned.  Figs.  168  and  169 
are  illustrations  of  enclosed  motors. 

For  ventilating  purposes  there  is  sometimes,  instead  of  a  pulley, 
a  fan  fixed  on  the  shaft  of  the  motor.  With  larger  fans  the  motor 


FIG.  170.— Motor  connected  to  Machine  Tool. 

is  fixed  on  the  case  of  the  ventilator.     Fig.  175  shows  a  big  fan 
combined  with  the  motor. 

Since  smaller  motors  are  generally  built  for  high  speeds,  it  is 
sometimes  necessary  to  reduce  these  speeds  by  means  of  reduction 
gears.  Even  with  belt  driving  it  is  sometimes  desirable  to  reduce 
the  speed  by  gearing.  Generally  the  reduction  gear  is  built 
together  with  the  motor,  and  on  the  slow  speed  countershaft  the 
coupling  or  the  belt  pulley  is  fixed. 


THE  ELECTRIC  MOTOR 


161 


Motors  in  America  are  used  for  a  wide  range  of  work. 

Fig.  170  shows  an  application  of  an  electric  motor  to  a  machine 
tool. 

Fig.    171   shows  an  application  to  a  pump. 

Fig.   172  shows  an  application  to  an  elevator. 

Fig.   173  shows  an  application  to  a  mine  hoist. 

On  battleships,  in  America,  a  very  extensive  use  of  motors  is 
made.  One  battleship  has  over  200  motors  installed  upon  it;  the 
turrets  are  turned  by  rnitor,  the  ammunition  raised  and  pushed  into 


FIG  171. — Motor  connected  to  Pump. 


the  guns,  boat  cranes  operated,  ventilating  blowers  driven,  and 
rudder  turned. 

A  special  extra  pole,  or  commutating  pole  motor,  has  recently  been 
developed.  It  has  a  magnetic  circuit,  as  shown  in  Fig.  174. 

This  figure  shows  a  4-pole  motor  of  usual  magnetic  circuit,  but  in 
addition  to  the  poles  A,  B,  C,  and  D  there  are  four  other  poles,  a', 
&',  c',  and  d' ',  which  are  wound  with  wire  in  series  with  the  armature, 
like  a  compound  motor  winding.  The  armature  is  wound  for  a  4-pole 
motor,  although  there  are  actually  eight  poles.  The  four  extra  poles 
are  about  half  the  size  of  the  regular  poles.  The  flux  from  these 
poles  is  in  such  a  direction  that  the  current  is  reversed  in  the  coil 
which  is  short  circuited  under  the  brush  without  shifting  brushes 
to  the  proper  pole  to  get  such  a  flux  (forward  in  a  generator  and 
backward  in  a  motor).  Thus,  such  a  motor  runs  without  shift  of 
brushes  and  in  either  direction  equally  as  well.  The  turns  on  the 


THE  ELECTRIC  MOTOR  163 

commutating  poles  are  chosen  so  that  just  the  right  amount  of  flux 


\ 


FIG.  173.— Application  to  Mine  Host. 

is  obtained  to  give  exact  reversal,  and  no  more;  hence,  such  motor 
run  sparklessly.  They  can  be  de- 
signed on  closer  lines  than  an  ordi- 
nary motor,  making  their  cost,  there- 
fore, less,  since  the  commutating 
poles  are  wound  with  series  spools. 
The  balance  of  flux  with  reversing 
requirements  of  the  armature  coils 
when  under  the  brush  is  the  samo 
at  all  loads.  Such  a  motor,  there- 
fore, gives  far  better  results  under 
overload  than  the  usual  design. 
This  extra  pole  application  is  just 
as  useful  for  generators,  and  from 
present  indications  the  commutat- 
ing pole  is  to  be  rapidly  extended 
in  dynamo  design. 


FIG.  174. — Four-pole  Inter-pole 
Magnetic  Circuit. 


164 


ELECTRICAL    ENGINEERING 


Electric  Traction 

An  important  application  of  electric  motors  is  that  for  rail  ways , 
especially  street  railways.  In  the  latter  case  the  current  supply 
device  consists  generally  of  a  hard-drawn  copper  wire,  which  is 


FIG.  175  — Electrically  driven  Fan     (Korting  Brothers). 

suspended  by  means  of  insulators  supported  either  by  posts  or  by 
cross  wires.  The  copper  wire  is  in  connection  with  the  positive  pole 
of  the  central  station  dynamo,  the  negative  pole  of  which  is 
connected  with  the  rails.  On  the  top  of  the  motor  car  there  is  fixed 


THE  ELECTRIC  MOTOR 


165 


FIG.  176.— Street-car  Motor,  closed. 


either  a  metal  bow,  or  an  iron  tube,  the  top  of  which  is  provided 
with  a  little  wheel,  the  "trolley."  The  bows  or  the  trolleys  are 
pressed  by  a  spring  arrangement  against  the  overhead  wire,  and 

serve  as  the  current  supply 
device.  Both  the  bows  and 
the  trolleys  are  very  well 
insulated  from  all  the  iron 
parts  of  the  car,  and  a  cable 
leads  from  them  to  the 
motor  starter  and  thence  to 
the  motor  itself.  The  latter 
is  fixed  beneath  the  car,  and 
drives  the  car-axle  by  means 
of  a  pinion  and  spur  wheel. 
The  second  pole  of  the  motor 
is  connected  with  the  car-axle, 
and  thus  through  the  wheels  and  the  rails  with  the  negative  pole 
of  the  central  station  dynamo.  Very  often  two  motors  are  used 

for  one  car,  each  of  which  drives  one 
axle. 

The  motors  employed  for  driving  elec- 
tric cars  have  generally  the  character- 
istic shape,  as  shown  in  Figs.  176  and 
177.  The  motor  is  entirely  enclosed 
to  prevent  dust  and  moisture  getting 
into  its  interior.  To  be  able  to  inspect 
the  commutator  and  the  brushes,  or  to 
take  out  the  bearings  or  the  armature, 
the  case  is  divided  into  two  parts, 
hinged  to  each  other;  the  upper  part 
may  be  fixed  and  the  lower  one  opened 
downwards,  or  mce  versa.  Since  it  is 
desirable  to  use  as  little  space  for  the 
motors  as  possible,  the  magnet  coils 
are  not  wound  on  separate  bobbins, 
but  are,  after  they  have  been  wound 
on  special  wooden  formers,  and  have 
been  well  insulated  with  impregnated 
cotton,  mica  and  so  on,  pushed  over  the 
cores.  Since  street  railways  are  gene- 
rally worked  with  a  voltage  of  500-600, 
all  the  motor  parts  must  be  excellently 
insulated. 

About  the  working  of  these  motors  nothing  special  has  to  be  re- 
marked. They  are  four-pole  motors  with  two  brush-holder  arms,  each 
of  which  is  provided  with  one  or  two  carbon  brushes.  The  motors 


FIG   177.— Motor,  open. 


166 


ELECTRICAL  ENGINEERING 


are  reversible,  and,  according  to  the  position  of  the  starting  lever, 
drive  the  car  forwards  or  backwards. 

The  starter  for  street-car  purposes  is  generally  called  a  controller. 
Since  it  has,  like  the  motors,  to  be  protected  against  dirt  and  dust, 
it  is  entirely  enclosed.  Its  internal  construction  (similar  to  that 
shown  in  Fig.  178)  is  entirely  different  from  that  of  the  starters 
with  which  we  have  hitherto  become  acquainted.  The  contact  pieces 


FIG.  178. — Controller  suitable  for  Single  Motor     (Royce,  Manchester). 

by  which  the  different  connections  are  effected,  are  not  arranged 
on  a  horizontal  base,  but  fixed  on  the  surface  of  a  vertical  cylinder. 
On  the  contact  pieces  or  contact  rings  there  are  sliding  brushes  or 
contact  levers  which  are  fixed  on  a  separate  wooden  plate,  whereas 
the  cylinder  with  the  contact  pieces  is  movable.  On  the  top  of  the 
cylinder  there  is  fixed  a  handle,  by  means  of  which  the  cylinder 
may  be  turned  to  different  positions.  By  rotating  the  cylinder  the 


THE  ELECTRIC   MOTOR  167 

connections  of  the  contact  rings  with  the  contact  levers  are  altered, 
as  with  the  usual  starting  apparatus. 

A  controller  for  a  single  motor-car  is  really  little  else  than  a 
common  starter.  It  has  a  position  of  rest,  marked  "stop,"  and  a 
number  of  starting  steps.  At  the  last  step  the  whole  of  the  starting 
resistance  is  short-circuited,  when  the  motor  is  switched  on  the  full 
voltage,  and  runs  at  full  speed.  The  reversing  of  the  motor  for  the 
opposite  direction  of  rotation  is  generally  effected  by  a  reversing 
switch,  which  is  separated  from  the  controller,  but  mechanically  con- 
nected with  the  latter  in  such  a  way  as  to  make  reversing  impossible, 
unless  the  motor  is  stopped. 

For  cars  which  are  provided  with  two  motors,  the  controller  becomes 
more  complicated.  In  this  case  there  are  two  main  working  positions, 
viz.,  firstly,  a  series  connection  of  the  motors,  when  each  motor  is 
switched  on  half  the  voltage  only;  and,  secondly,  parallel  connection 
of  the  two  motors,  when  each  motor  is  switched  on  the  full  voltage, 
thus  running  twice  as  fast  as  before.  Starting  the  motors  is  effected 
by  connecting  first  of  all  the  two  series  .connected  motors  in  series 
with  some  resistance,  and  then  gradually  short-circuiting  this 
resistance.  The  motors  then  run  with  half  the  voltage  and  a 
corresponding  speed.  But  if  the  connection  is  altered,  and  the 
motors  connected  in  parallel,  they  run  with  full  speed. 

Similar  controllers  are  also  employed  for  electric  cranes. 

Fig.  179  shows  the  cylinder  of  an  American  street-car  controller 
for  two  motors,  developed  on  a  flat  surface  to  show  the  contacts 
more  easily. 

Another  form  of  controller  mechanism  is  that  known  as  the  mul- 
tiple-unit control  system.  In  this  case  the  controller  is  split  up  into 
its  component  parts,  each  being  separate  from  the  other,  but  operated 
from  a  master  controller,  which  excites  magnets,  or  contactors,  located 
upon  the  component  parts  just  at  the  right  time,  so  that  they 
take  their  turn  in  closing  or  opening  the  current  circuits  operating 
the  motors  on  the  cars,  as  the  master  controller  regulates.  These 
contactors,  made  to  open  heavy  currents,  may  be  placed  under  the  car 
out  of  the  way  where  room  is  available,  and  where  the  big  arc  resulting 
from  their  breaking  large  currents  can  give  no  trouble.  The  master 
controller,  on  the  other  hand,  having  only  to  direct  the  small  current 
necessary  to  operate  the  magnets  of  the  contactors,  takes  but  little 
room  and  can  be  placed  conveniently  to  the  motorman.  In  addition, 
if  several  cars  are  connected  together,  all  equipped  with  motors  and 
contactors,  one  master  controller  can  operate  them  all  simultaneously. 
The  figure  shows  the  lines  from  the  master  controller  to  the  contactor 
for  one  car.  As  many  cars  can  be  connected,  in  parallel  to  this  same 


168 


ELECTRICAL  ENGINEERING 


controller,  as  desired.     Thus,  a  whole  train  may  be  operated  from 
one  or  more  master  controllers,  and  every  axle  helps  the  train  along. 


0       17617       0   ||_€ 

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- 

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|00  [^17619 

0    17620  0    ||    (2 

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17619  >-*|0  0|              I7<: 

322->  0        0 

i*44'*i 

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017621  0 

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*-  153"  M 

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I4O*  -     —  *- 

FIG.  179.— Cylinder  Development  of  Street-car  Controller  (Two  Motors). 


In  this  way  a  very  fast  acceleration  may  be  obtained.  This  system  is 
much  used  in  America  on  elevated  trains  and  on  large  surface  cars  and 
locomotives,  and  is  generally  known  as  the  multiple-unit  control.  By 
its  introduction  a  great  stride  was  made  in  electric  traction,  as  un- 
limited power  can  be  controlled  by  splitting  it  up  into  easily  handled 
units. 

Following  is  a  detailed  description  of  the  multiple-unit  system 
adopted  by  the  General  Electric  Company  of  the  United  States  of 
America. 


THE  ELECTRIC   MOTOR  169 

The   Sprague=General    Electric    Type    M   Control 

System 

The  Sprague-General  Electric  Type  M  Control  is  designed  pri- 
marily for  the  operation  of  a  train  of  motor-  and  trail-cars,  coupled  in 
any  combination,  and  the  whole  operated  as  a  single  unit  from  any 
controller  on  the  train.  The  system  may  also  be  used  to  advantage 
on  individual  equipments  and  locomotives. 

The  control  apparatus  for  each  motor-car  may  be  considered  as 
consisting  essentially  of  a  motor  controller  and  a  master  controller. 

The  motor  controller  comprises  a  set  of  apparatus — usually  located 
underneath  the  car — which  handles  directly  the  power  circuits  for 
the  motors,  connecting  them  in  series  and  parallel  and  commutating 
the  starting  resistance  in  series  with  them.  This  motor  controller  is 
operated  electrically,  and  its  operation  in  establishing  the  desired 
motor  connections  is  controlled  by  the  motorman  by  means  of  the 
master  controller,  which  is  similar  in  construction  to  the  ordinary 
cylinder  controller,  and  is  handled  in  the  same  manner.  Instead  of 
effecting  the  motor  combinations  directly,  however,  this  controller 
merely  governs  the  operation  of  the  motor  controller. 

The  master  controller  operates  a  number  of  electrically  operated 
switches  or  "  contactors/7  which  close  and  open  the  various  motor 
and  resistance  circuits,  and  an  electrically  operated  "reverser"  that 
connects  the  field  and  armature  leads  of  the  motors  to  give  the 
desired  direction  of  movement  to  the  car.  Both  the  contactors  and 
reverser  are  operated  by  solenoids,  the  operating  current  for  which 
is  admitted  to  them  by  the  master  controller. 

Each  motor-  and  trail-car  is  equipped  with  train  cable,  consisting 
of  nine  or  ten  individually  insulated  conductors  connected  to  corre- 
sponding contacts  in  coupler  sockets  located  at  each  end  of  the  car. 
This  train  cable  is  connected  identically  on  each  motor-car  to  the 
master-controller  fingers  and  the  contactor  and  reverser  operating 
coils,  and  is  made  continuous  throughout  the  train  by  couplers  between 
cars,  connecting  together  corresponding  terminals  in  the  coupler 
sockets. 

All  wires  carrying  current  supplied  directly  from  the  master  con- 
troller form  the  " control  circuit";  those  carrying  current  for  the 
motors  form  the  " motor"  or  " power  circuit." 

Inasmuch  as  the  motor-controller  operating  coils  are  connected 
to  this  control  train  line,  it  will  be  appreciated  that  energizing  the 
proper  wires  by  means  of  any  master  controller  on  the  train  will 
simultaneously  operate  corresponding  contactors  on  all  the  motor- 
cars and  simultaneously  establish  similar  motor  connections  on  all 
cars. 


170  ELECTRICAL  ENGINEERING 


ADVANTAGES 

The  Sprague-General  Electric  Type  M  Control  permits  a  train  of 
motor-cars  and  trailers  to  be  operated  as  a  single  unit  from  any  master 
controller  on  the  train.  If  desired,  a  master  controller  can  be  placed 
on  each  platform  of  trail-cars,  thereby  providing  for  the  operation  of 
the  train  from  any  platform.  With  this  arrangement  the  motorman 
can  be  always  at  the  head  of  the  train,  regardless  of  the  combination 
of  the  cars. 

The  entire  train,  equipped  with  Type  M  Control,  may  thus  be 
regarded  as  a  unit;  the  motorman  has  the  same  control  over  a  train 
that  he  would  have  over  a  single  car  with  the  ordinary  cylinder  con- 
troller. 

Should  the  motorman  remove  his  hand  from  the  operating  handle 
of  the  master  controller,  the  current  will  be  immediately  cut  off  from 
the  entire  train,  thus  diminishing  the  danger  of  accident  in  case  the 
motorman  should  suddenly  become  incapacitated. 

The  system  will  operate  at  any  line  potential  between  300  and 
600  volts,  and  the  action  of  all  contactors  is  absolutely  reliable  and 
instantaneous. 

On  heavy  equipments  the  effort  of  the  motorman  in  operating  the 
master  controller  is  so  much  less  than  that  required  to  handle  a  large 
cylindrical  controller  that  he  can  give  more  attention  to  the  air- 
brakes and  other  parts  of  the  equipment,  especially  in  cases  of  emer- 
gency. The  ease  with  which  it  is  operated  also  makes  the  Type  M 
Control  particularly  well  suited  for  use  on  large  locomotives. 

The  approximate  total  weight  per  motor-car  of  control  equipments, 
exclusive  of  supports,  is  as  follows: 

Aggregate  H.P.  of  Motors.  "Weight  of  Equipment  in  Pounds. 
100  1500 

200  2000 

300  2500 

500  4000 

640  4500 

The  approximate  weight  of  the  apparatus  for  each  trail-car,  which 
comprises  train  cable,  coupler  sockets  and  connection  boxes,  is  100 
pounds. 

In  many  cases  it  will  be  found  advantageous  to  anticipate  the 
future  growth  of  an  interurban  road  by  equipping  each  motor-car 
with  Type  M  Control.  In  these  cases  it  will  be  easy  to  change  from 
single  car  to  train  service  whenever  warranted  by  traffic  conditions. 

The  position  of  the  handle  on  that  master  controller  which  the 
motorman  is  operating  always  indicates  the  position  of  motor-control 
apparatus  on  all  cars. 


172  ELECTEICAL  ENGINEERING 

On  account  of  the  great  flexibility  of  this  system,  it  can  be  readily 
adapted  to  many  classes  of  service  other  than  that  of  train  operation. 
The  small  space  occupied  by  the  master  controller  and  the  ease  with 
which  the  controller  may  be  operated  make  this  system  for  heavy 
hoists  desirable  in  some  cases,  or  other  classes  of  severe  direct-current 
service  requiring  a  controller  easily  manipulated,  or  one  which  may 
be  located  at  a  considerable  distance  from  the  motors. 

All  parts  subject  to  wear  are  readily  replaceable. 


CONTACTORS 

The  contactors  are  the  means  of  cutting  in  and  out  the  various 
resistances,  of  making  and  breaking  the  main  circuit  between  trolley 
and  motors,  and  of  changing  from  series  to  parallel  connection. 

Each  contactor  consists  of  a  movable  arm  carrying  a  renewable 
copper  tip  which  makes  contact  with  a  similar  fixed  tip,  and  a  coil 
for  actuating  this  arm  when  supplied  with  current  from  the  master 
controller.  The  contactor  is  so  designed  that  the  motor  circuit  is 
closed  only  when  current  is  flowing  through  its  operating  coil;  and 
gravity,  assisted  by  the  spring  action  of  the  finger,  causes  the  arm 
to  drop  and  open  this  circuit  immediately,  when  the  control  circuit 
is  interrupted.  Each  contactor  has  an  effective  and  powerful  mag- 
netic blow-out,  which  will  disrupt  the  motor  circuit  under  conditions 
far  exceeding  normal  operation.  In  closing,  the  copper  tips  come 
together  with  a  wiping  action,  which  cleans  and  smooths  their  surfaces. 

All  contactors  in  an  equipment  are  practically  identical,  and  the 
few  parts  which  are  subject  to  burning  and  wear  are  so  constructed 
as  to  be  readily  replaceable. 

In  order  to  save  space  arid  eliminate  interconnections  as  much 
as  possible,  several  contactors  are  mounted  on  the  same  base.  The 
contactors  should  preferably  be  located  under  the  car,  and  boxes  are 
therefore  supplied  which  facilitate  installation,  protect  the  contactors 
from  brake-shoe  dust  and  other  foreign  material,  and  provide  the 
necessary  insulation.  These  boxes  are  built  with  perforated  openings 
for  ventilation,  but  shields  are  supplied  for  closing  these  perforations 
whenever  desirable. 

REVERSER 

The  general  design  of  the  reverser  is  somewhat  similar  to  the 
ordinary  cylindrical  motor  reversing;  switch,  with  the  addition  of 
electro-magnets  for  throwing  it  to  either  forward  or  reverse  position. 
In  general  construction,  the  operating  coils  are  similar  to  those  used 
on  the  contactors,  but  in  order  to  secure  absolute  reliability  of  action 
in  throwing,  the  coil  is  given  full  line  potential.  The  reverser  is 


THE  ELECTRIC  MOTOR  173 

provided  with  small  fingers  for  handling  control  circuit  connections, 
and,  when  it  throws,  the  operating  coil  is  disconnected  from  the  ground 
and  is  placed  in  series  with  a  set  of  contactor  coils,  thus  cutting  the 
operating  current  down  to  a  safe  running  value.  These  coils  are 
protected  by  a  fuse,  which  will  immediately  open  the  circuit  if  the 
reverser  fails  to  throw.  If  the  position  of  the  reverser  does  not 
correspond  to  the  direction  of  movement  indicated  by  the  reverse 
handle  on  the  master  controller,  the  motors  on  that  car  cannot  take 
current.  While  the  motors  are  taking  current  the  operating  coil  is 
energized,  and  the  electrical  circuits  are  interlocked  to  prevent  pos- 
sibility of  throwing. 


MASTER   CONTROLLER 

The  master  controller  is  considerably  smaller  than  the  ordinary 
street-car  controller,  but  is  similar  in  appearance  and  method  of  oper- 
ation. Separate  power  and  reverse  handles  are  provided,  as  ex- 
perience has  led  to  the  adoption  of  this  arrangement  in  preference 
to  providing  for  the  movement  of  a  single  handle  in  opposite  directions. 

An  automatic,  safety,  open-circuiting  device  is  provided,  whereby, 
in  case  the  motorman  removes  his  hand  from  the  master-controller 
handle,  the  control  circuit  will  be  automatically  opened  by  means  of 
auxiliary  contacts  in  the  controller,  which  are  operated  by  a  spring 
when  the  button  in  the  handle  is  released.  This  device  is  entirely 
separate  and  distinct  in  its  action  from  that  of  the  main  cylinder. 
Moving  the  reverse  handle  either  forwards  or  backwards  makes  con- 
nections for  throwing  the  reverser  to  either  forward  or  backward 
position.  The  handle  can  be  removed  only  in  the  intermediate  or 
off  position.  As  the  power  handle  is  mechanically  locked  against 
movement  when  the  reverse  handle  is  removed,  it  is  necessary  for 
the  motorman  to  carry  only  this  handle  when  leaving  the  car. 

When  the  master  controller  is '  thrown  off,  both  line  and  ground 
connections  are  severed  from  the  operating  coils  of  important  con- 
tactors, and  none  of  the  wires  in  the  train  cable  are  alive. 

The  current  carried  by  the  master  controller  is  about  2.5  amperes 
for  each  equipment  of  400  H.P.  or  less.  This  small  current  carrying 
capacity  permits  a  compact  construction,  and  the  controller  weighs 
only  130  pounds. 

MASTER-CONTROLLER   SWITCH 

A  small  enclosed  switch  with  magnetic  blow-out  is  used  to  cut  off 
current  from  each  master  controller,  and  is  supplied  with  a  small 
cartridge  fuse  enclosed  in  the  same  box.  When  this  switch  is  open 


174  ELECTRICAL  ENGINEERING 

all  current  is  cut  off  from  that  particular  master  controller  which  it 
protects. 

CONTROL   CABLE 

A  special  flexible  cable,  made  up  of  different  colored  individually 
insulated  conductors,  is  used  for  the  train  cable  and,  whenever  pos- 
sible, to  make  connections  between  the  various  pieces  of  control 
apparatus. 

CONNECTION  Box 

Connection  boxes  are  provided  for  connecting  the  control  circuit 
cables  at  junction  points  without  splicing,  and  small  copper  terminals 
.are  supplied  for  attaching  to  the  ends  of  the  individual  conductors. 

CONTROL   COUPLERS 

The  master-control  cables  of  each  car  terminate  in  sockets  and 
are  interconnected  by  means  of  a  short  section  of  similar  flexible 
cable  fitted  with  plugs.  Each  socket  contains  a  number  of  insulated, 
metallic  contacts  connected  to  the  train  wires,  and  the  terminal 
plugs  of  the  coupler  contain  corresponding  contacts.  The  parts 
subject  to  wear  are  readily  replaceable. 

All  coupler  sockets  are  provided  with  spring  catches  which  hold 
the  plugs  in  contact  under  normal  conditions,  and  permit  them  to 
automatically  release  in  case  two  cars  separate. 

CONTROL   CUT-OUT  SWITCH 

This  is  a  switch,  usually  nine  point,  installed  on  each  motor-car, 
and  is  used  to  disconnect  the  operating  coils  of  the  contactors  and 
reverser  from  the  train  cable,  and  hence  render  them  inoperative. 

CONTROL  FUSES 

On  each  car  several  small  enclosed  fuses  are  placed  in  the  control 
circuit  at  such  points  as  to  effectively  protect  the  apparatus. 

CONTROL  RHEOSTAT 

During  acceleration,  tubes  of  a  high-resistance  rheostat  are  con- 
nected in  series  with  the  contactor  coils  to  cut  down  the  operating 
current  to  a  value  approximating  that  for  the  running  positions  of 
the  controller.  This  rheostat  is  enclosed  in  a  sheet  iron  case  for 
protection. 


THE  ELECTRIC  MOTOR  175 

CIRCUITS 

The  motor  circuit  is  local  to  each  car,  and  on  the  first  point 
the  current  on  entering  from  the  trolley  or  third-rail  shoe  passes 
through  the  following  pieces  of  apparatus  in  the  order  named:  main 
switch  and  fuse,  contactors,  resistances,  reverser,  motors;  thence  to 
ground. 

In  the  control  circuit,  the  course  of  the  current  from  trolley  to 
ground  is  through  the  master-controller  switch  and  fuse,  master  con- 
troller, connection  box,  to  the  cut-out  switch.  From  the  cut-out 
switch  the  current  passes  through  the  control  cable  to  the  operating 
coils  of  the  reverser  and  contactors,  and  thence  through  fuses  to 
ground. 

AUTOMATIC  FEATURES 

The  apparatus  described  is  used  with  the  standard  equipment  for 
hand  control.  If  automatic  features  are  desired,  certain  minor 
changes  will  be  entailed. 

INSTALLATION 

In  order  to  insure  economical  operation,  it  is  essential  that  the 
apparatus  should  be  so  located  under  the  car  as  to  be  easily  inspected 
and  repaired.  Attention  should  therefore  be  given  to  the  disposition 
of  the  apparatus.  The  best  results  are  obtained  by  first  locating  the 
contactors  and  reverser  to  the  best  possible  advantage.  The  air- 
brake apparatus  can  be  placed  in  the  remaining  space. 

The  Electric  Brake 

With  street  railway-cars  it  is  of  the  greatest  importance  to  be  able 
to  apply  a  brake  quickly,  especially  in  cases  of  danger,  when,  for 
instance,  people  are  in  the  way  of  the  car.  The  usual  mechanical 
braking,  as  used  for  horse-cars,  is  not  sufficient  for  the  heavier  and 
more  quickly  running  electric  car.  A  very  effective  kind  of  braking 
may  be  effected  by  disconnecting  the  motor  from  the  mains,  and  then 
connecting  the  armature  brushes  with  each  other  through  a  resistance. 

Let  us  consider,  first  of  all,  a  shunt  motor,  assuming  the  shunt 
coils  to  be  connected  with  the  outer  mains  during  the  whole  running. 
If  now  we  disconnect  the  armature  from  the  latter,  connecting  the 
brushes  to  a  resistance,  the  motor  will,  as  long  as  it  is  rotating, 
act  as  a  dynamo.  The  armature  will  deliver  a  current  into  the 
resistance,  which  current  is  greater  the  quicker  the  armature  is 
running,  and  the  smaller  is  the  resistance.  It  is  quite  clear  that 
for  the  production  of  this  current  mechanical  work  has  to  be  spent. 
Thus  the  live  energy,  which  the  car  still  has  after  switching  off  the 
oiotors,  is  spent  in  generating  a  current,  and  will  soon  be  consumed. 


176 


ELECTRICAL  ENGINEERING 


The  car  will  therefore  run  slower  and  slower,  just  as  if  the  wheels 

had  been  braked  mechanically.     This  kind  of  braking  is  especially 

effective  at  a  very  high  speed  of  the  car, 

whereas  at  low  speeds  it  is  much  less  so.        [ 

For  absolutely  stopping  a  car,  this  kind 

of  braking   cannot  be   employed  at  all, 

since  there  is  only  a  braking  effect  if  a 

current  is  really  generated,  and  the  latter 

can   only   occur   when   the   armature   is 

rotating.     Each  electric  car  has  therefore, 

besides  the  electric  braking  arrangement, 

to  be  provided  \vith  a  mechanically  acting 

one. 

When  a  car  is  provided  with  series 
motors,  which  is  generally  the  case  with 
street  railways,  a  reversal  of  the  magnet 
connections  is  required  for  getting  a 
braking  effect.  Imagine  the  motor  to  be 
connected  according  to  Fig.  181,  the 
current  flowing  in  the  armature  in  the  direction  from  I.  to  II.,  in 
the  magnets  from  V.  to  VI.  As  we  know,  a  back  E.M.F.  is 
produced  in  the  armature,  which,  after  disconnecting  the  latter  from 
the  mains,  would  tend  to  produce  a  current,  leaving  the  armature 
in  I.,  and  entering  it  again  in  II.  Hence  if,  for  the  purpose  of 
braking  the  motor,  we  simply  insert  a  resistance  between  I.  and  VI. 


FIG.  181. — Running  Position 
of  Series  Motor. 


FIG.  182. — Incorrect  Connect:on          FIG.  183. — Correct  Connection  for 
'  for  Braking:  Braking. 


(see  Fig.  182),  then  the  current  will  leave  the  armature  in  I.,  flow 
through  the  resistance,  through  the  magnet  coils  in  the  direction 
from  VI.  to  V.,  and  enter  the  armature  at  II.  The  current  will 
therefore  flow  through  the  magnet  coil  in  a  direction  opposite  to 
that  before.  The  magnetism  of  the  machine  will  be  destroyed, 
no  current  production  is  possible,  and  the  braking  effect  will 


THE  ELECTRIC  MOTOR  177 

instantly  cease.  To  get  a  braking  effect,  we  must  connect  the 
magnet  coils  so  as  to  cause  the  armature  current  to  flow  through 
them  in  the  same  direction  as  when  the  machine  was  working  as 
motor.  The  proper  brake  connections  for  a  series  motor  are  shown  in 
Fig.  183. 

The  brake  connections  are  generally  executed  by  means  of  the 
controller.  From  the  "stop"  position  to  the  left  of  the  driver  the 
different  running,  and  to  his  right  the  brake  positions  of  the 
controller  handle,  are  generally  arranged. 


The  Magnetic  Blow-Out 

A  controller  has  generally  far  harder  work  to  do  than  a  common 
starter,  since  it  has  continually  to  be  operated  for  starting  the  motor, 
altering  its  speed,  and  braking.  To  prevent  the  arcs  and  sparks, 
arising  from  the  frequent  disconnections  made  with  the  controller, 
from  destroying  the  contact  rings  and  the  brushes,  it  is  necessary  to 
blow  out  the  sparks  quickly.  This  may  be  done  by  magnetic  blow- 
outs. 

If  we  bring  a  strong  magnet  near  an  electric  arc,  we  observe 
that  the  arc  is  deflected,  being  bent  hi  a  large  bow,  and  finally 
extinguished.  The  arc  is  an  easily  movable  conductor.  It  consists 
of  glowing  metal  or  carbon  vapour,  through  which  the  electric 
current  flows.  As  we  know,  each  movable  electric  conductor  is 
deflected  by  a  magnetic  field,  and  therefore  the  deflection  and 
rupturing  of  the  electric  arc  may  be  understood. 

Each  controller  is  provided  with  a  strong  electro-magnet,  the 
effect  of  its  magnetic  field  extending  over  the  fixed  contact  brushes. 
The  sparks  arising  between  these  contact  brushes  and  the  contact 
rings  are  hence  quickly  extinguished. 


178 


ELECTRICAL  ENGINEERING 


Operating  Troubles  with  Direct=current  Motors 

Fig.    184    shows    a   contactor  equipped  with  a  magnetic  blow- 


FIG.  184— Contactor. 


FIG.  185. — Master  Controller. 


out  to  extinguish  the  arc,  and  Fig.   185  shows  a    master  controller 
operating  many  of  these  contactors. 


CHAPTER  V 

ACCUMULATORS 

WE  have  learned  in  the  first  chapter  of  this  book,  that  if  with  the  help 
of  metal  plates  we  pass  a  current  through  acidulated  water,  a  decom- 
position results,  with  the  separation  of  hydrogen  and  oxygen. 

Another  phenomenon  also  takes  place  with  which  we  have  not  yet 
dealt.  If  the  resistance  of  the  voltmeter  and  the  pressure  at  its  ends 
be  first  measured,  and  from  the  values  so  obtained  we  calculate  the 
current  which  ought  to  flow  through  the  circuit  in  accordance  with 
Ohm's  Law,  we  shall  find  that  what  is  actually  measured  by  an 
ammeter  is  far  smaller. 

The  explanation  lies  in  the  fact  that  in  addition  to  the  E.M.F. 
driving  the  current  through  the  liquid  there  is  a  back  or  counter 
E.M.F. ,  just  as  we  have  learnt  is  the  case  with  an  electro-motor. 

At  the  positive  electrode,  which  is  the  place  of  entrance  of  the 
current,  the  oxygen  is  liberated,  whilst  at  the  negative  the  hydrogen 
is  evolved.  The  current  flows  in  the  liquid  from  the  positive  to  the 
negative  pole;  the  back  E.M.F.,  on  the  other  hand,  is  so  directed 
that  it  tends  to  send  a  current  in  the  liquid  from  the  negative  to  the 
positive  pole.  Whenever  a  current  passes  through  a  liquid,  as  in  the 
case  of  galvanic  cells,  the  development  of  gas  at  the  plates  produces  a 
back  E.M.F.,  which  tends  to  weaken  the  working  pressure  of  the  cell. 
This  effect  is  called  electrolytic  polarization.  The  simplest  element 
with  copper  and  zinc  in  dilute  sulphuric  acid  shows  the  property 
of  polarization  in  a  very  marked  manner,  and  causes  the  E.M.F.  of 
such  a  cell  to  rapidly  diminish  when  the  cell  is  in  use. 

The  separation  of  oxygen  and  hydrogen  brings  about  a  chemical 
alteration  of  the  immersed  metal  plates,  unless  they  are  made  of 
metals  like  platinum.  For  example,  if  we  use  as  electrodes  plates 
of  iron,  then  the  oxygen  liberated  at  the  positive  pole  will  cause 
oxidation  of  the  iron.  We  know  that  iron  rusts  on  account  of 
the  oxygen  in  the  air  which  combines  with  the  metal.  The  com- 
pound so  produced  is  known  to  chemists  as  oxide  of  iron.  Exactly 
the  same  thing  happens  during  electrolysis,  when  the  positive  plate 
is  of  iron.  If  we  had  used  lead  instead  of  iron  a  corresponding 
change  takes  place.  On  the  surface  of  the  positive  electrode  a  layer 
of  lead  oxide  would  appear. 

179 


180  ELECTRICAL   ENGINEERING 

At  the  negative  electrode  hydrogen  is  liberated,  but  does  not,  as  a 
rule,  attack  the  electrode.  If  lead  were  used  it  would  remain  bright, 
or,  if  it  had  previously  been  covered  with  a  thin  film  of  oxide,  this 
will  now  be  destroyed,  because  the  hydrogen,  having  an  attraction  for 
oxygen,  will  decompose  the  oxide,  producing  water  and  liberating 
lead. 

After  the  passage  of  the  current  we  have  no  longer  two  similar 
electrodes,  but  at  the  positive  pole  we  have  the  metal  coated  with 
lead  oxide,  and  at  the  negative  a  clean  lead  plate.  Two  different 
metals  in  a  liquid  give,  as  we  are  aware,  an  E.M.F.  The  origin  of 
the  back  E.M.F.  will  now  be  self-evident.  When  gas  is  evolved,  this 
collecting  on  the  plates  gives  a  further  difference  between  the  plates. 
Hence,  to  get  a  current  through  the  liquid  its  voltage  must  be 
sufficient  to  overcome  the  back  E.M.F.  The  voltage  of  such  a  cell 
may  amount  to  more  than  2  volts,  i.e.  twice  as  much  as  the  E.M.F. 
of  a  simple  galvanic  cell. 

When  a  cell  is  coupled  to  an  outside  source  of  pressure,  so  as  to 
send  a  current  through  it,  the  process  of  charging  is  said  to  be  in 
progress.  On  stopping  the  current  the  evolution  of  gas  immediately 
ceases,  but  if  we  now  connect  the  poles  of  the  cell  by  a  wire, 
a  current  is  obtained,  the  direction  of  which  is  opposite  to  the 
charging  current,  and  the  cell  is  said  to  discharge.  It  may  again 
be  charged  by  coupling  it  to  a  pressure  supply,  then  discharged,  and 
the  process  may  be  repeated  as  often  as  may  be  desired. 

An  apparatus  used  in  this  way  is  called  an  accumulator — that  is, 
a  storage  arrangement  which  is  capable  of  accumulating  energy  and 
giving  it  back  when  desired. 

The  accumulator  that  we  have  so  far  considered  can  supply 
current  for  a  short  time  only,  for  in  charging  it,  only  the  surface  of 
the  plates,  which  is  in  direct  connection  with  the  liquid,  can  be 
chemically  altered.  When  this  is  effected,  further  charging  is  use- 
less. The  liquid  is  then  decomposed,  but  the  oxygen  formed  on  the 
positive  plate,  after  the  whole  surface  has  been  oxidized,  is  unable  to 
further  penetrate  into  the  plate,  and  escapes,  therefore,  in  the  form  of 
bubbles.  Hence,  if  the  charging  be  continued,  the  only  effect  will  be 
to  decompose  the  liquid. 

If  however,  after  the  first  charge,  the  accumulator  is  again 
discharged,  then  the  plate  has  been  mechanically  altered.  The  surface 
of  the  lead  plate  has  become  spongy,  and  if  we  charge  the  accumu- 
lator again,  the  chemical  change  can  penetrate  a  little  deeper  into  the 
plate.  If  these  charges  and  discharges  be  continued,  the  " capacity" 
of  the  accumulator  is  gradually  increased. 

This  process  of  forming  the  plates  was  first  used  by  the  French 
experimenter  Plante,  and  plates  made  in  this  way  are  called  Plante 
plates.  The  process  of  formation  is  a  very  lengthy  one,  and  must  be 
continued  for  weeks,  and  even  months  if  the  cell  is  to  have  much 


ACCUMULATORS 


181 


capacity.  The  process  is  completed  when  one  plate  is  covered  to  a 
good  depth  with  finely  divided  lead,  and  the  other  has  a  corresponding 
amount  of  lead  peroxide. 

It  was  again  a  Frenchman — Faure — who  showed  how  the  tedious 
process  of  forming  might  be  much  diminished  in  time.  Instead  of 
using  pure  lead  plates,  he  applied  to  the  lead  a  mixture  of  the  two 
oxides  of  lead  called  minium.  If  as  positive  electrode  a  lead  grid 
filled  with  minium  be  employed,  and  as  negative  electrode  a  pure 
lead  plate,  or  a  similar  grid  plate  having  the  holes  filled  with  spongy 


mt%8s&$®& 


:^ 


FIG.    186.— Accumulator  Plate. 


lead,  then  we  have  from  the  beginning  two  chemically  different 
electrodes,  which  have  already  an  E.M.F.,  and  do  not  require  a 
formation,  or  in  some  cases  a  formation  lasting  only  a  short  time. 

In  the  manufacture  of  accumulators  it  is  extremely  important 
that  the  greatest  care  be  taken  that  the  pasted  substance  is  se- 
cured firmly  to  the  lead  grid.  Various  methods  have  been  devised  for 
locking  the  material  within  the  openings  of  the  lead  grids,  one 
example  being  shown  in  Fig.  186. 

In  order  to  give  the  Plante  accumulators  great  capacity,  it  is 
essential  that  the  acting  surface  be  as  great  as  possible.  Now,  the 
working  surface  of  a  plate  of  any  particular  size  may  be  increased 


182 


ELECTRICAL   ENGINEERING 


by  providing  it  with  very  many  ridges  and  cavities  such  as  shown  in 
Fig.  187. 

The  chemical  processes  which  take  place  in  the  accumulator  are 
by  no  means  as  simple  as  we  have  hitherto  assumed  them  to  be.  We 
have  supposed  that  the  water  only  in  the  cell  is  decomposed,  and 
that  the  acid  serves  merely  for  the  purpose  of  improving  the  conduc- 
tivity of  the  water.  The  sulphuric  acid  really  has  an  influence  on 
the  chemical  process,  and  it  has  been  found  that  the  proportion  of  the 


FIG.   187. — Accumulator  Plate. 


quantity  of  water  to  that  of  the  acid  is  of  great  importance.  The 
complete  chemical  changes  which  take  place  during  the  charging  and 
discharging  of  an  accumulator  are  too  complicated  to  be  described 
here.  The  main  facts  are  these: — During  charging,  spongy  lead  is 
formed  at  one  plate  which  is  termed  by  cell  makers  the  negative 
plate;  whilst  at  the  other  plate,  called  the  positive,  a  dark  red  oxide 
called  lead  peroxide  is  formed.  On  discharging  the  cell  both  the 
lead  and  lead  peroxide  are  changed  to  lead  sulphate. 

Under  normal  conditions  an  accumulator  should  never  be  dis- 


ACCUMULATORS  183 

charged  so  that  its  E.M.F.  falls  lower  than  1.80  to  1.83  volts.  For 
the  purpose  of  charging,  a  higher  pressure  is  necessary.  Soon  after 
the  beginning  of  the  charge  the  E.M.F.  of  a  cell  rises  to  two  volts, 
and  when  completely  charged  becomes  2.5  to  2.7  volts.  When  gas  is 
evolved  from  both  plates  of  the  accumulator,  the  end  of  the  charge 
is  indicated,  and  that  no  more  oxygen  is  being  absorbed  by  the 
positive  plate. 

The  rising  of  the  voltage  during  charging  is  firstly  due  to  the 
chemical  change  of  the  plates,  and  then  for  two  other  reasons.  There 
are  formed  bubbles  of  hydrogen  and  oxygen,  which  increase  the  back 
E.M.F.  of  the  accumulator,  and  further  the  accumulator  has  of  course 
an  ohmic  resistance,  to  overcome  which  requires  the  expenditure  of 
certain  E.M.F. 

After  charging,  the  voltage  of  the  accumulator  falls  immediately, 
down  to  2.2  volts.  If  we  connect  the  accumulator  terminals  with  an 
outer  resistance,  so  that  it  supplies  current,  its  terminal  voltage 
will  fall  still  further,  and  more  so  the  greater  the  current  supplied. 
This  is  partly  due  to  the  chemical  alteration  of  the  plates,  and  partly 
due  to  the  ohmic  resistance  of  the  cell.  In  charging  we  have  to 
make  the  terminal  voltage  larger  than  the  E.M.F.  of  the  accumulator 
in  order  to  overcome  the  ohmic  resistance.  But,  in  discharging,  the 
voltage  drop  caused  by  the  ohmic  resistance  takes  away  part  of  the 
E.M.F.,  so  that  the  terminal  voltage  becomes  smaller  than  the  E.M.F. 
of  the  cell.  Hence  the  ohmic  loss  works  to  our  disadvantage,  both 
in  charging  and  discharging. 

From  an  accumulator  we  cannot  therefore  get  as  much  energy 
as  we  put  into  it.  The  ratio  between  the  quantity  of  energy  which 
we  get  in  discharging  to  that  energy  which  has  to  be  spent  for 
charging  is  called  the  efficiency  of  the  accumulator.  With  good 
accumulators  this  is  about  80  per  cent.  For  100  units  of  work  put 
into  the  accumulator  we  get  about  80  units  from  the  accumulator,  the 
remaining  20  units  are  transformed  into  chemical  action  and  heat. 

The  accumulator  is  an  excellent  means  for  storing  electrical 
energy.  If  at  any  time  there  is  electrical  energy  at  liberty,  we  may 
charge  the  accumulator,  and  afterwards  obtain  electrical  energy  from 
it.  We  may,  for  instance,  charge  an  accumulator  during  10  hours 
with  2  amps.,  and  then  take  20  amps,  during  nearly  1  hour;  or  we  may 
charge  it  with  20  amps,  during  1  hour,  and  then  obtain  nearly  2  amps, 
during  10  hours.  The  accumulator  may  be  compared  to  a  savings  bank, 
to  which  we  may  pay  money  from  time  to  time  in  pence,  and  get  back 
in  one  payment  a  large  sum:  or  to  which  we  may  pay  at  one  trans- 
action a  large  sum,  to  be  withdrawn  in  small  amounts  as  desired. 
We  may  also,  of  course,  charge  and  discharge  the  accumulator  exactly 
at  the  same  rate. 

This  convenient  transformation,  of  which  we  have  just  spoken, 
is,  of  course,  limited.  The  current  passed  into  or  taken  out  of  an 


1S4 


ELECTRICAL  ENGINEERING 


accumulator  should  never  exceed  a  certain  amount,  or  the  plates  will 

be  injured.  The  current  an  accumu- 
lator can  stand  depends  chiefly  on  the 
size  of  the  plates.  To  obtain  large 
currents  with  plates  of  reasonable  size, 
they  are  not  made  in  one  piece,  but 
consist  of  several  plates  connected  in 
parallel.  Figs.  188  and  189  are 
illustrations  of  accumulator  cells,  each 
consisting  of  several  plates.  They  are 
placed  side  by  side,  so  that  any  positive 
plate  lies  between  two  negative  ones, 
and  any  negative  (except  the  plates  at 
the  two  ends)  between  two  positive 
plates.  Thick  lead  rods  are  used  to 
connect  all  the  positive  plates  together, 
and  in  a  similar  way  all  the  negative 
plates  are  in  connection.  The  voltage 

of  the  cell  is,  of  course,  equal  to  one  consisting  of  two  plates  only. 


FIG.  188. -Storage  Cell. 


FIG.  189.— Storage  Cell     (General  Electric  Co.). 


ACCUMULATORS 


185 


To  get  the  resistance  as  small  as  possible  the  plates  are  placed 
very  near  each  other.  Direct  contact  of  the  plates  is  prevented  by 
glass  or  rubber  rods  placed  between  them. 

Since  in  most  cases  far  higher  pressures  than  2  volts  are 
employed,  a  number  of  accumulator  cells  are  placed  in  series,  forming 
an  accumulator  battery.  The  single  cells  have  then  to  be  connected 
so  that  the  positive  terminal  of  the  first  cell  is  connected  with  the 
negative  terminal  of  the  second;  and  so  on  in  series.  Since  any 
other  metal  would  be  attacked  and  destroyed  by  the  sulphuric 


FIG.  190. — Portable  Storage  Battery 

acid  or  the  spray  arising  from  it,  lead  strips  or  rods  are  used  for 
connecting  the  poles.  In  Fig.  190  a  battery  is  shown  which 
consists  of  four  cells,  mounted  in  a  wooden  box,  which  is  lined 
with  celluloid. 

If  the  battery  has  to  feed  110  volt  lamps,  it  must  consist  of  about 
60  cells ;  for,  as  we  have  learnt,  accumulators  are  discharged  down  to  a 
voltage  of  about  1.80  to  1.83,  so  that  finally  the  voltage  of  60  series- 
connected  cells  becomes  110. 

If,  on  the  other  hand,  we  had  the  lamps  continuously  switched 
on  the  whole  battery,  this  would  be  a  great  fault,  for  the  voltage  of  a 
single  cell  is  at  the  beginning  of  the  discharge  more  than  two  volte. 
Thus  the  lamps  at  the  beginning  getting  more  than  120  volte,  would 


186  ELECTRICAL  ENGINEERING 

be  overrun,  and  have  a  short  life  only.  Further,  the  lamps  would 
burn  very  brightly  at  the  beginning,  and  darker  later  on.  To  prevent 
this  the  lamps  must  at  the  beginning  of  the  discharge  be  in  connection 
with  a  smaller  number  of  cells.  If  the  cell  voltage  be,  for  instance,  2, 

then  55  cells  at  first  are  sufficient; 
then,  as  the  discharge  continues,  56, 
57,  etc.,  cells  are  necessary  to  give 
the  voltage  of  110.  If  the  voltage 
per  cell  falls  to  1.83,  then  all  the 
60  cells  will  be  required. 

To  easily  secure  this  variation  of 
pressure,  cell  switches,  as  diagram- 
matically  shown  in  Fig.  191,  are 
employed.  From  each  of  the  last 
cells  a  cable  leads  to  a  number  of 

contacts,    which   are   arranged   in    a 
FIG.  |191 —Cell  Switch.  circle>  and  oyer  which  a  metal  lever 

slides.  The  lamps  are  between  cne 

pole  of  the  battery,  and  the  lever  of  the  battery  switch.  If  the 
latter  is  in  its  extreme  left  position,  the  lamps  are  connected  to  the 
least  number  of  cells.  The  cells  on  the  right  are  then  without  effect, 
they  do  not  supply  any  current.  In  moving  the  lever  to  the  right  to 
the  next  contact,  one  of  the  cells  previously  not  in  use  is  switched 
into  the  circuit.  As  the  battery  voltage  decreases  the  lever  is  moved 
more  and  more  to  the  right,  and  finally  all  the  cells  are  in  use. 

As  may  be  seen  from  the  above,  the  end  cells  are  not  used  so 
much  as  the  others,  and  therefore  it  is  not  necessary  to  charge  them 
as  long  as  the  main  cells.  The  battery  switch  may  therefore  also  be 
used  with  advantage  for  charging  the  cells.  First  of  all,  the  cells  are 
connected  in  series,  the  lever  of  the  battery  switch  covering  the  last 
contact.  At  the  last  cell,  which  is  discharged  but  little,  a  strong 
development  of  gas  will  soon  be  observed.  If  by  means  of  a  volt- 
meter we  examine  the  voltages  of  the  single  cells,  the  last  will 
probably  show  2.5,  whereas  the  voltage  of  the  other  cells  will  still  be 
lower,  probably  2.3.  The  last  cell  is  then  switched  off  by  removing 
the  lever  of  the  battery  switch  to  the  last,  contact  but  one.  When 
the  last  cell  but  one  becomes  fully  charged,  the  lever  is  again  moved 
back  one  contact.  Thus  we  see  that  on  charging  it  is  necessary  to 
move  the  lever  gradually  to  the  left,  whereas  on  discharging  we  must 
move  the  lever  to  the  right  as  required. 

A  battery  recently  developed  in  America  by  Thomas  A.  Edison 
has  iron  for  its  plates  and  for  solution  hydrate  of  sodium  or  caustic 
soda.  The  chemical  action  here  is  that  of  oxidation,  just  as  with 
lead  plates.  The  life  of  this  battery  is  reported  to  be  far  longer  than 
the  lead  battery,  and  its  weight  per  horse-power  less.  Many  difficulties 
have  been  met  and  overcome  in  manufacture,  and  while  the  sales 
have  not  been  large,  those  produced  have  given  very  great  satisfaction. 


ACCUMULATORS  187 


Machines  for  charging  Accumulators 

For  charging  a  battery  of  110  volts  a  pressure  of  60  X  2.5  =  150 
volts  is  required;  sometimes  the  voltage  per  cell  has  to  be  raised  up 
to  2.7—2.75,  bringing  the  voltage  of  the  whole  battery  up  to  165.  A 
dynamo  employed  for  charging  such  a  battery  must  therefore  be 
built  for  a  far  higher  voltage  than  that  used  on  the  lighting 
circuits. 

For  charging  accumulator  batteries,  shunt  dynamos  are  generally 
employed.  We  know  that  with  these  machines  by  means  of  a  shunt 
regulator  it  is  possible  to  alter  the  voltage  within  certain  limits. 
Machines  for  charging  accumulators  are  now  built  so  that  by  means 
of  a  large  shunt-regulating  resistance  their  voltage  can  be  varied 
between  110  to  160. 

Series  and  compound  dynamos  are  practically  never  used  for 
charging  accumulators.  With  a  series  dynamo  the  voltage  increases 
with  the  current.  Hence  if,  by  any  means — say  by  a  resistance 
inserted  in  the  circuit  instead  of  the  cells — we  get  such  a  voltage  on 
the  dynamo  as  to  get  a  certain  current  in  the  cells,  then,  on  switching 
in  the  latter,  the  E.M.F.  of  the  accumulators  will  increase.  The 
result  will  be  that  the  difference  between  the  E.M.F.  of  the  dynamo 
and  the  back  E.M.F.  of  the  accumulators,  and  hence  the  current,  will 
decrease.  Due  to  this  smaller  current,  the  E.M.F.  of  the  series 
dynamo  will  now  fall,  and  it  might  then  happen  that  a  current  will 
flow  back  from  the  battery  to  the  dynamo,  and  reverse  its  polarity. 
The  use  of  a  series  dynamo  is  therefore  impossible. 

Compound  dynamos  are,  for  similar  reasons,  also  unsuitable  for 
directly  charging  accumulators.  If,  however,  the  voltage  of  the 
compound  dynamo  be  higher  than  the  maximum  accumulator  voltage, 
then  with  such  a  machine  the  cells  may  be  charged  by  employing  a 
series  resistance.  In  this  case  it  is  not  to  be  feared  that  the 
dynamo  voltage  may  fall  so  low  that  a  current  will  be  sent  back 
from  the  accumulators  through  the  resistance  to  the  machine. 

The  most  suitable  and  nearly  exclusively  employed  machine  .for 
charging  accumulators  is  the  shunt  dynamo.  For,  firstly,  with 
decreasing  current  its  voltage  increases,  and  it  can  therefore  hardly 
happen  that  its  voltage  should  fall  below  that  of  the  battery;  and, 
secondly,  even  if  this  should  take  place  (for  instance,  through  the 
driving  steam-engine  running  more  slowly),  a  current  would  flow  from 
the  battery  into  the  machine  in  an  opposite  direction  through  the 
armature  only.  The  magnet  coils  are  in  this  case  traversed  by  a 
current  flowing  in  the  same  direction  as  before,  the  only  difference 
being  that  the  current  comes  from  the  cells  instead  of  from  the 


188 


ELECTRICAL  ENGINEERING 


Discharge 


armature.  Thus  the  polarity  of  the  magnets  is  not  reversed,  and 
the  reversal  of  the  current  does  not  cause  any  subsequent  injurious 
effect,  as  it  would  do  with  series  or  compound  dynamos. 

With  the  means  with  which  we  have  so  far  become  acquainted, 
it  would  not  be  possible  to  employ  a  dynamo  for  lighting  and 

charging  accumulators  simultaneously. 
With  two  battery  switches  there  is  no 
difficulty  in  doing  this.  In  Fig.  192  it 
is  shown  how  the  last  few  cells  of  the 
battery  are  connected  with  the  contacts 
of  two  cell  switches.  The  one  which  is 
below  in  the  diagram  is  called  the 
charging  battery  switch,  the  upper  one 
the  discharging  switch. 

The  machine  is  connected  with  the 
first  cell  and  the  charging  lever;  the 
lamps  are  connected  with  the  first  cell 
and  with  the  discharging  lever.  Now 
we  may  produce  with  the  machine  a 
voltage  of  150,  and  charge  with  this 
voltage  the  60  series-connected  cells. 
The  charging  lever  covers,  for  instance, 
the  last  contact,  but  the  discharging 
lever  is  removed  far  to  the  left,  so  that 
probably  only  44  cells  are  switched  on 
the  lamp  mains.  If  the  voltage  of  each 
cell  be  2.5,  then  the  44  cells  will  just 
give  the  proper  lighting  voltage  of  110. 
The  battery  is — in  a  manner — fully 
charged  and  simultaneously  partly  discharged.  If,  for  instance,  we 
charge  with  100  amps.,  and  the  lamps  consume  10  amps,  only,  then 
the  full  current  of  100  amps,  will  flow  through  those  cells  only 
which  are  between  the  charging  and  discharging  levers.  All  the 
other  cells  are  charged  with  100  - 10  =  90  amps.  only. 

As  a  matter  of  fact  the  lighting  current  of  10  amps,  is  not 
supplied  by  the  battery,  but  by  the  dynamo.  Wre  have  in  this  case 
a  branched  circuit.  From  the  positive  pole  of  the  machine  a  current 
of  100  amps,  is  flowing  to  the  charging  lever,  giving  a  current  through 
the  additional  cells  only.  Through  these  the  full  current  is  flowing. 
When  the  current  comes  to  that  cell  which  is  connected  with  the 
contact  just  covered  by  the  discharging  lever,  the  current  has  two  ways 
— one  through  the  whole  battery  to  the  negative  pole,  the  other  one 
through  the  discharging  lever  and  the  lamps  to  the  negative  pole. 
In  the  latter  the  branch  currents  are  again  combined,  flowing  back 
to  the  negative  brush  of  the  dynamo. 

This  arrangement  is  only  employed  in  cases  where  the  current 


FIG.  192.— Double  Cell 
Switch. 


ACCUMULATORS 


189 


supplied  to  the  mains  during  charging  is  small  compared  with  the 
charging  current  of  the  battery,  otherwise  a  far  larger  current  will 
flow  through  the  end  cells  than  through  the  others.    The  former 
must    then    be    mad:)    much 
larger  than  the  latter,  or;  owing 
to    the   larger    currents,    they 
will   be   far  sooner   destroyed 
than  the  other  cells. 

Sometimes,  for  raising  the 
voltage  during  charging  a 
special  machine,  a  so-called 
booster,  is  employed.  The 
main  dynamo  then  supplies  the 
normal  voltage,  and  it  can 
therefore,  during  charging, 
supply  current  to  the  mains. 
With  the  main  dynamo  the 
booster  is  in  series,  by  con- 
necting the  negative  brush  of 
the  latter  with  the  positive 
brush  of  the  main  dynamo  (see 
Fig.  193).  The  battery  is 
connected  to  the  negative  pole 
of  the  main  dynamo  and  the 
positive  pole  of  the  booster. 
The  booster  can  be  regulated 
from  a  very  low  voltage  up  to  about  50  volts  (provided  we  have  a 
main  voltage  of  110,  and  60  cells). 

When  we  commence  to  change  the  cells  the  booster  has  to  supply 
a  very  low  voltage,  the  excitation  is  very  weak,  but  the  charging 
current  might  probably  be  very  large.  In  such  a  case  it  may  happen 
that,  owing  to  the  armature  reaction,  which  weakens  the  magnetism, 
the  polarity  of  the  machine  is  changed.  To  prevent  this,  the  booster 
is  generally  separately  excited. 


FIG.  193. — Connections  for  Booster 
when  charging. 


Battery  Switch 


Fig.  194  shows  one  type  of  an  accumulator  battery  cwitch.  We 
see  from  this  illustration  that  the  contact  pieces  are  arranged  in 
a  circle,  .and  that  a  lever  with  an  elastic  brush  slides  on  them. 
The  lever  is  not  quite  as  simple  as  are  those  of  regulating  resistances. 
We  observe  a  spiral  of  wire  on  it,  and  that  there  are  fixed,  not  one, 
but  two  contact  brushes  on  the  lever.  If  we  had  a  single  contact 


190  ELECTRICAL  ENGINEERING 

brush  only,  then  there  would  be  two  possibilities :  the  brush  may 
be  either  narrower  or  wider  than  the  insulating  space  between  the 
contact  pieces.  If  we  make  the  brush  narrower,  then  in  moving 
the  lever  a  break  in  the  current  takes  place.  This  will  cause  a 
violent  sparking,  and,  if  the  motion  of  the  lever  be  a  slow  one,  the 
lamps  will  be  extinguished.  Certainly  with  a  quick  motion  of  the 
lever  the  current  would  not  be  entirely  interrupted,  nevertheless  a 
nickering  of  the  lamps  would  be  caused,  and  the  contacts  burnt  by 
the  sparks.  If,  on  the  other  hand,  the  contact  brush  be  so  wide 
as  to  touch  the  second  contact  before  it  has  left  the  first  one,  then  the 
flickering  would,  of  course,  disappear;  but  at  the  moment  the  brush 


FIG.  194.— Battery  Switch     (Voigt  &  Haffner). 

covers  two  contact  pieces,  one  of  the  cells  is  entirely  short-circuited 
thus  causing  a  strong  current,  which  would  damage  the  cells.  To 
avoid  this,  near  the  narrow  main  brush  which  serves  for  taking 
the  current,  another  auxiliary  brush  is  provided,  which  is  con- 
nected with  the  main  brush  by  means  of  a  resistance  spiral  of 
German  silver  or  nickelin,  but  is  insulated  from  the  lever.  If 
the  lever  stops,  then  only  the  main  brush  covers  the  contact,  the 
auxiliary  brush  is  on  the  insulating  piece  between  the  contacts, 
and  thus  is  without  effect.  If  we  move  the  lever,  in  order  to 
switch  in  or  off  a  cell,  then,  before  the  main  brush  has  left  the  first 
contact,  the  auxiliary  brush  covers  the  next  one.  The  cell  being  now 
between  the  two  contacts  is.  of  course,  connected  with  a  closed  circuit, 


ACCUMULATORS  191 

but  is  not  ,s/ior/-circuited,  for  the  resistance  of  the  spiral  is  in  the 
circuit,  and  if  this  resistance  be  made  sufficiently  large,  the  current 
produced  by  the  cell  will  be  only  a  moderate  one.  At  the  next 
moment  the  main  brush  leaves  the  first  contact,  but  since  the 
auxiliary  brush  now  covers  the  second  contact,  there  cannot  be 
a  further  interruption  of  the  current,  but  the  resistance  spiral 
is  inserted  in  the  outer  circuit.  If  we  left  this  resistance  con- 
tinuously switched  in  the  circuit,  energy  would  be  wasted,  for  the 
resistance  spiral  consumes  nearly  as  much  voltage  as  is  supplied 
by  the  last  added  cell.  Hence  we  must  remove  the  lever  a  little 
more,  until  the  main  brush  covers  the  contact  piece,  and  the 
auxiliary  brush  stands  again  on  the  insulating  piece.  In  this  way 
both  an  interruption  of  the  current  and  a  short  circuit  is  avoided,  and 
on  moving  the  lever  violent  sparking  is  prevented. 

The  same  effect  may  be  obtained  by  connecting  each  cell  with 
two  contact  pieces,  having  one  of  them  connected  directly,  and  the 
other  one  through  a  resistance  spiral.  A  brush  of  double  width 
may  then  slide  over  the  contacts.  Battery  switches  of  this  construc- 
tion have  therefore  twice  as  many  contacts  as  those  of  the  type 
previously  considered. 

Charging  and  discharging  switches  may  be  combined  in  a  single 
apparatus,  the  double-cell  switch.  There  is  no  difference  in  the 
arrangement  of  the  contact  pieces  between  this  and  a  single-cell 
switch,  but  there  are  two  levers  insulated  from  each  other,  and 
having  arms  of  different  length,  sliding  over  the  contact  pieces. 
This  obviously  produces  exactly. the  same  effect  as  if  we  had  con- 
nected each  of  the  end  cells  with  the  contacts  of  two  different  cell 
switches. 

In  some  cases  it  is  desirable  to  save,  during  the  time  current 
is  taken  from  the  battery,  any  attendance.  In  such  cases  an 
automatic  cell  switch  may  be  employed  with  great  advantage.  The 
arrangement  consists  chiefly  of  a  small  motor,  which,  by  a  relay,  is 
switched  in  so  as  to  run  either  clock-  or  counter-clockwise,  according 
to  the  voltage  increasing  above  or  decreasing  below  the  normal  value. 
The  contact  brush  of  the  cell  switch  is.  by  the  motion  of  the  motor, 
then  moved  either  upwards  or  downwards.  When  the  normal  voltage 
is  reached,  the  motor  is  switched  off  by  the  relay. 


Accumulator  Apparatus 

We  have  previously  learned  that  when  machines  and  accumulators 
work  in  parallel  the  current  from  the  latter  may  possibly  flow  back 
into  the  dynamos.  We  have  further  learnt  that  this  danger  is  least 
with  shunt  dynamos ;  but  in  this  case,  also,  a  reversal  of  the  current  is 


192 


ELECTRICAL   ENGINEERING 


liable  to  damage  the  accumulators.  Assuming,  for  instance,  that  the 
steam-engine  driving  the  dynamo  runs  somewhat  slower,  then  there  may 
come  a  moment  in  which  the  E.M.F.  of  the  dynamo  is  smaller  than 
that  of  the  accumulators.  The  dynamo  will  then  consume  current, 
and  run  as  a  motor  driving  the  steam-engine.  Hence  the  accumulators 
are  discharged  with  a  current,  which  may  be  dangerously  large. 
To  prevent  this  minimum  cut-outs  are  provided  in  the  accumulator 
circuit.  Fig.  195  shows  such  an  apparatus.  There  are  two  cups 
filled  with  mercury,  into  which  can  dip  a  piece  of  metal,  U-shaped 
and  fixed  on  a  movable  lever.  The  latter  has  an  iron  axle,  with 
which  two  small  iron  rods  are  connected,  which  project  backwards. 
These  iron  rods  are  connected  by  a  brass  strip,  on  which  is  fixed 


FIG.   195. — Minimum  Cut-out  (The  Electrical  Company}. 

a  counter-weight,  pulling  downwards  the  back  part  of  the  apparatus , 
thus  tending  to  lift  the  U-shaped  piece  out  of  the  mercury.  Over 
the  iron  axle  a  copper  spiral  is  wound,  one  end  of  which  is  connected 
with  the  inner  mercury-basin,  the  other  end  with  one  main  terminal. 
The  outer  mercury-cup  is  in  connection  with  the  second  main 
terminal.  If  a  current  flows  through  the  copper  spiral,  both  the 
iron  axle  and,  the  two  iron  rods  projecting  backwards  become 


ACCUMULATORS  193 

magnetized,  the  whole  arrangement  representing  then  a  horseshoe 
electro-magnet.  Imagine  now  one  terminal  to  be  connected  with 
the  machine,  the  other  one  with  the  accumulators,  then,  in  the 
position  of  the  apparatus  shown  in  the  figure,  no  current  can  flow 
through  the  spiral.  If,  now,  we  wish  to  charge  the  accumulators,  we 
have  to  bring  the  dynamo  to  a  voltage  which  is  larger  by  a  few  volts 
than  that  of  the  accumulators.  Then  we  have  to  lift  the  counter- 
weight of  the  minimum  cut-out,  so  that  the  U-shaped  metal  piece 
dips  into  the  two  mercury-cups,  and  the  limbs  of  the  electro-magnet 
knock  against  the  iron  bar.  In  doing  so  we  close  the  dynamo  circuit. 
The  current  coming  from  the  dynamo  flows  from  the  right  main 
terminal  to  the  mercury-cup,  through  the  U -shaped  metal  bridge 
(which  is  insulated  from  the  lever)  to  the  second  mercury-cup,  from 
there  through  the  copper  spiral,  and  from  the  latter  to  the  second 
terminal  of  the  apparatus  and  to  the  accumulators.  The  current 
magnetizes  the  horseshoe-shapen  iron  pieces  of  the  movable  part,  so 
that  it  sticks  to  the  iron  bar  above.  With  a  current  over  a  certain 
value,  this  attraction  is  so  great  as  to  overcome  the  effect  of  the 
counter-weight,  and  to  keep  the  movable  part  in  this  position. 
If,  however,  the  dynamo  voltage  falls,  then,  at  the  moment  in 
which  the  E.M.F.  of  the  accumulators  is  equal  to  the  chanring 
pressure,  the  current  flowing  in  the  circuit  will  be  nil.  and  .  the 
electro-magnet  of  the  movable  part  will  lose  its  magnetism.  It 
no  longer  keeps  fast  the  iron  bar,  and  the  counter-weight  will 
lift  the  iron  bridge  from  the  mercury-cups,  thus  disconnecting  the 
circuit. 

If  the  electro-magnet  touches  the  bare  iron  keeper,  then,  owing  to 
the  remanent  magnetism,  the  proper  working  of  the  armature  is  pre- 
vented. To  avoid  this,  two  brass  screws  are  provided,  which  project 
over  the  electro-magnet,  thus  preventing  the  armature  from  direct 
contact  with  the  electro-magnets. 

In  some  cases,  instead  of  the  minimum  cut-outs,  maximum  cut- 
outs are  employed.  These  act  when  the  current  exceeds  a  certain 
amount.  The  electro-magnet,  excited  by  a  current  passing  around 
the  spiral,  causes,  in  this  case,  by  attracting  the  iron  armature,  a  break 
in  the  circuit. 

Maximum  cutouts,  both  for  accumulators  and  for  motors,  prevent 
them  from  being  heavily  overloaded.  As  we  know,  fuses  are  generally 
selected  so  that  they  will  melt  with  a  current  of  double  the  normal 
value.  Since,  now,  this  increased  current  cannot  do  the  mains  any 
harm,  but  may  in  some  cases  seriously  damage  the  motors  or  cells, 
a  maximum  cut-out  is  desirable,  which  is  so  adjusted  that  when 
the  allowable  current  is  exceeded,  the  circuit  is  automatically  dis- 
connected. 

We  have  still  to  mention  the  current  indicators,  which  are 
generally  inserted  in  the  accumulator  circuit.  When  the  current 


194 


ELECTRICAL   ENGINEERING 


flows  through  the  accumulators  in  such  a  direction  as  to  charge 
them,  the  pointer  of  the  instrument  indicates  "charge;"  if  the 
current  flows  in  an  opposite  direction  the  pointer  indicates  "  dis- 
charge." See  Fig.  196. 

If  in  the  accumulator  circuit  be 
inserted  a  Deprez  ammeter,  with 
the  zero  in  the  middle,  as  shown 
in  Fig.  197,  a  current  indicator 
becomes  superfluous. 

To  examine  the  state  of  the 
single  cells,  a  little  voltmeter  is 
used  with  a  range  of  3  volts,  one 


FIG.  196. — Current  Indi- 
cator (General  Electric 
Co). 


FIG.  197.— Ammeter  with  Central 
Zero  Point  (Berend  &  Co). 


FIG.   198.— Voltmeter  for  Cell  Testing 

(Berend  &  Co.}. 


terminal  of  which  terminates  in  a  point;  the  other  end  is  connected 
with  a  short  cable,  the  end  of  which  also  terminates  in  a  point.  The 
two  points  are  then  pressed  against  the  positive  and  negative 
electrodes  of  the  cell  respectively,  and  so  the  cell  voltage  is  measured. 
An  accumulator  tester  of  this  kind  is  shown  in  Fig.  198. 


ACCUMULATORS  195 


Applications  of  Accumulators 

Accumulators  serve  many  purposes.  In  central  stations  for  small 
towns  the  current  consumption  is  considerable  only  during  a  few  hours 
of  the  day,  whereas  during  the  remaining  time  comparatively  few  lamps 
are  in  use.  It  is  then  very  uneconomical  to  run  the  machines  during 
the  whole  day  and  night.  If  the  machines  run  during  the  day,  when 
the  demand  for  current  is  little,  they  are  under  a  small  load. 
Dynamos  and  steam-engines  running  with  a  small  load  have  a  low 
efficiency.  Whilst,  for  instance,  with  a  full-loaded  steam-engine 
plant  the  coal  consumption  per  useful  kilowatt-hour  varies  between 
3J  and  6  pounds  (according  to  the  size  of  the  plant),  this  consumption 
may  with  machines  that  are  very  little  loaded  increase  up  to 
12—20,  and  even  more,  pounds.  But  if  an  accumulator  be  used,  this 
may  be  charged  during  some  hours  of  the  day,  enabling  the  machines 
to  run  with  greater  load.  During  the  whole  time  of  small  demand 
of  current  the  accumulator  alone  is  sufficient.  The  machines  are 
stopped  during  this  time,  and  are  started  again  during  the  time  of 
maximum  demand,  when  they  can  be  assisted  by  the  battery,  so  that 
at  the  time  of  the  maximum  demand  a  larger  current  may  be  supplied 
to  the  mains  than  could  be  delivered  by  the  machines  themselves. 

In  factories  where  electricity  is  used  both  for  lighting  and  power 
transmission,  during  the  working  hours  much  current  is  consumed  by 
the  electric  motors,  and,  in  addition,  in  the  evening  a  large  current  is 
needed  for  lighting  workshops,  office-rooms,  etc.  After  the  work- 
ing hours  but  little  current  is  required  for  lighting  special  rooms, 
corridors,  yards,  etc.  This  current  is  then  supplied  by  the  accumu- 
lators, which  may  be  charged  again  during  the  working  hours.  If 
the  battery  is  of  sufficient  capacity,  even  motors  for  driving  small 
lathes  and  other  tools  may,  after  the  end  of  the  general  work,  be 
provided  with  current  from  the  battery. 

Excellent  service  may  be  rendered  by  accumulators  as  buffer 
batteries  when  working  in  parallel  with  dynamos.  In  central  electric 
stations,  both  for  lighting  and  traction  purposes-,  shunt  dynamos 
are  frequently  employed.  These  dynamos  have,  as  we  know,  the 
property,  that  with  an  increasing  current  the  terminal  voltage 
decreases.  Now,  in  all  central  stations  in  which  electro-motors  are 
installed  on  the  mains,  sudden  rushes  of  current  occur,  due  to  the 
switching  in  or  sudden  loading  of  motors.  This  causes  a  sudden  fall 
of  the  voltage.  Now  the  shunt  regulator  cannot  be  worked  so 
quickly  as  to  prevent  fluctuation  of  the  lamps  on  the  network. 
The  same  thing  takes  place  if  the  load  is  suddenly  thrown  off 
the  dynamos.  In  this  case  the  voltage  increases  rapidly.  Again, 


196  ELECTRICAL   ENGINEERING 

with  central  stations  for  traction  purposes  it  may  happen  that  the 
current  consumption  increases  for  short  periods  to  three,  four,  or 
even  five  times  the  average  demand,  as  when  several  cars  start 
simultaneously.  This  will  cause  the  voltage  of  the  dynamo  sup- 
plying the  current  to  suddenly  fall  an  undesirable  amount.  If, 
however,  there  is  a  battery  of  suitable  size  working  in  parallel  with  the 
dynamo,  then  it  will  supply  current  at  these  times  of  sudden  demand, 
and  prevent  the  voltage  of  the  mains  from  falling  lower  than  that  of 
the  battery.  When  a  great  number  of  amperes  no  longer  is  needed, 
the  E.M.F.  of  the  dynamo  will  tend  to  rise,  and  the  battery  will 
now  be  charged.  In  the  opposite  case  when  the  current  consumption 
increases  beyond  the  normal  output  of  the  dynamo,  the  battery  will 
again  supply  current  to  the  mains.  The  battery  thus  is  ready  for 
any  sudden  rushes,  and  acts  just  like  a  buffer  between  dynamo  and 
network.  The  dynamo  will,  therefore,  if  running  in  parallel  with 
a  battery,  work  with  a  far  steadier  load,  and  thus  prove  more 
economical  than  without  the  battery. 

^  If  there  be  water  power  of  comparatively  small  amount,  then  we 
might  accumulate  in  a  battery  energy  during  the  whole  day,  and 
during  the  evening  take  a  comparatively  large  amount  of  energy 
from  the  battery.  This  system  is  frequently  used  for  lighting 
purposes. 

In  the  cases  hitherto  dealt  with,  the  batteries  have  been  stationary. 
In  many  cases  portable  accumulators  are  employed,  both  for  lighting 
and  power  purposes.  For  lighting  railway  cars,  for  instance,  accumu- 
lators are  charged  at  a  terminus,  and  put  in  a  special  box  be- 
neath the  car.  From  them  glow  lamps  can  be  supplied  with  the 
necessary  current  to  light  the  car. 

When  it  is  wished  to  avoid  trolley  wires  in  streets,  the  car  can 
be  provided  with  accumulators,  which  may  be  charged  at  special 
charging  stations.  If  only  at  certain  parts  of  a  line  trolley  wires  are 
not  allowed  to  be  used,  a  combined  system  is  possible.  The  car 
is  then  provided  both  with  accumulators  and  trolley  equipment. 
On  some  parts  of  the  line  the  current  is  taken  from  the  overhead 
trolley  wires,  and  at  the  same  time  the  accumulators  are  also 
charged  with  this  current.  Along  the  other  parts  of  the  line  the 
accumulators  supply  the  necessary  electrical  energy  to  the  motors. 

Since  storage  batteries  with  a  great  capacity  have  a  considerable 
weight,  accumulator  cars  are  generally  far  heavier  than  cars  with 
motors  only.  It  may  also  happen  that,  if  the  battery  is  not 
sufficiently  charged,  or  the  state  of  the  street  on  account  of  dirt, 
snow,  etc.,  is  very  difficult  for  traction,  the  car  may  be  brought  to 
a  stop,  because  the  battery  is  exhausted.  For  this  reason  neither  the 
accumulator  alone,  nor  the  combined  system  is  very  reliable,  and  it 
is  more  satisfactory  to  have  a  special  underground  system  whenever 
the  overhead  system  cannot  be  employed. 


ACCUMULATORS  197 

Motor  cars  may  also  be  driven  electrically,  and  provide  an 
extended  application  for  portable  batteries. 

The  same  remark  applies  to  boats  and  launches.  The  spindle 
of  the  screw  is  coupled  directly  with  the  motor,  and  the  latter  is  fed 
by  a  battery. 

The  accumulator  has  a  considerable  advantage  over  primary 
galvanic  cells.  It  can,  by  charging,  be  restored  to  its  former  state, 
whereas  with  primary  cells  this  is  not  possible. 

The  student  must  clearly  understand  that  the  accumulator  does 
not  store  electrical  energy.  It  stores  chemical  energy  which  is  con- 
verted into  electrical  energy  when  the  cell  is  discharged,  the  electrical 
power  depending  on  the  rate  of  discharge.  We  must  carefully 
distinguish  between  electrical  power  and  electrical  energy.  For  the 
former  a  convenient  unit  is  the  watt,  for  the  latter  the  unit 
mentioned  on  p.  195,  the  kilowatt-hour  is  in  commercial  use. 


CHAPTER   VI 


WORKING    OF  DIRECT-CURRENT  DYNAMOS  IN  PARALLEL 

ALTHOUGH  dynamos  are  built  of  great  output  (2000  kilowatts  and 
over),  it  is  seldom  the  case  that  there  is  only  a  single  dynamo  erected 
for  the  whole  output  of  a  central  station,  but  generally  two  or  more 
dynamos  are  used,  each  of  which  has  to  supply  a  part  of  the  total 
output.  ^This  division  of  the  plant  is  for  several  reasons.-  First  of 
all,  continuity  of  service  must  be  maintained.  If  an  accident 


FIG.  199. — Shunt  Dynamos  ready  for  Switching  in  Parallel. 

happens  to  one  of  the  machines,  there  are  still  others  to  maintain,  to 
a  certain  extent,  the  demand.  Secondly,  it  is  possible  to  run  one  or 
more  machines  nearly  fully  loaded,  as  required,  and  hence  they  will 
work  at  the  highest  efficiency. 

When  several  dynamos  are  used,  it  is  usual  to  arrange  them  in 
parallel  on  the  same  network.     For  this  purpose  shunt  dynamos  are 

198 


WORKING  OF  DYNAMOS  IN  PARALLEL  199 

suitable  (see  Fig.  199)  as  well  as  compound.     It  is  almost  impossible 
to  combine  series  dynamos  in  this  way. 

Imagine,  for  instance,  two  series  dynamos  working  in  parallel; 
these  would  alter  their  voltage  continuously,  according  to  their  load. 
Assuming  that  their  E.M.F. >s  were,  at  a  definite  load,  just  alike; 
then,  with  an  increasing  load,  their  voltages  would  rise.  It  is,  however, 
not  at  all  certain  that  these  pressures  will  rise  equally.  At 
the  double  load,  the  voltage  of  one  dynamo  may  increase  20  per 
cent.,  whereas  that  of  the  other  one  perhaps  only  15  per  cent.  But 
at  the  same  moment  the  latter  machine  supplies  less  current,  hence 
its  armatures  will  instantly  lose  or  nearly  lose  its  voltage.  The 
result  will  be  that  the  current  flows  through  it  from  the  other 
dynamo,  and,  being  in  an  opposite  direction,  reverses  its  poles.  The 
same  thing  may  take  place  without  a  variation  of  the  load  if  the 
speed  of  one  of  the  dynamos  decreases. 

With  shunt  dynamos  we  know  that  the  voltage  varies  also  accord- 
ing to  the  load,  but  in  an  opposite  way.  The  voltage  increases  on 
decreasing,  and  decreases  on  increasing,  the  load.  If,  at  a  given 
time,  the  load  be  equally  divided  between  two  machines  and  then 
the  load  is  suddenly  decreased,  it  is  also  possible  here  that  the  E.M.F. 
of  one  dynamo  is  greater  than  that  of  the  other,  so  that  the  E.M.F.  of 
the  first  dynamo  rises,  say  from  110  to  113,  whereas  that  of  the  second 
dynamo  changes  from  110  to  112  volts.  This  will,  however,  have 
only  the  consequence  that  the  first  machine  will  supply  more  current 
than  the  second  one  until  the  E.M.F.  of  the  first  machine  is  equal 
to  that  of  the  second  one.  Even  assuming  that  the  second  machine, 
due  to  the  slower  speed  of  the  driving  engine,  remains  with  its 
E.M.F.  so  low  as  no  longer  to  supply,  but  to  consume,  electrical 
energy,  this  will  only  cause  the  other  machine  to  be  overloaded. 
The  dynamo  taking  current  will  now  run  as  a  motor,  but  a  reversal 
of  the  poles  docs  not  occur,  because  the  current  flows  around  the 
magnets  in  the  same  direction  as  before. 

It  is  quite  another  matter  with  compound  dynamos.  If  with 
these  a  reversal  of  the  current  happened,  great  inconvenience  would 
arise,  since  the  reversed  current,  flowing  through  the  series-coil, 
would  weaken  the  magnetism,  which  is  produced  chiefly  by  the 
shunt-coil.  A  means  of  avoiding  this  by  the  use  of  a  so-called 
equalizing  wire  (the  connection  of  which  with  the  poles  of  the 
dynamos  is  shown  in  Fig.  200)  has  been  devised.  This  equalizing 
wire  must  be  connected  with  those  poles  of  the  dynamos  with  which 
are  also  connected  the  ends  of  the  series  windings.  As  long  as  both 
armatures  have  the  same  voltage  the  current  will  not  flow  through 
the  equalizing  wire,  but  only  through  the  two  mains. 

Now  let  us  consider  what  will  take  place  if,  by  any  accident,  the 
E.M.F.  of  one  machine  becomes  lower  than  that  of  the  other  machine 
so  that  it  now  consumes,  instead  of  delivers,  electrical  energy. 


200 


ELECTRICAL  ENGINEERING 


Assume  that  machine  II.  is  taking,  and  machine  I.  is  supplying, 
current,  then  the  current  will  flow  from  the  negative  terminal  of 
machine  I.  through  the  equalizing  wire,  through  machine  II.  (in  an 
opposite  direction  to  that  when  supplying  current),  and  through  the 
positive  bus-bar  back  to  the  positive  brush  of  machine  I.  Thus 
a  current  flows  in  an  opposite  direction  through  the  equalizing 


FIG.  300. — Compound  Dynamos  ready  for  Switching  in  Parallel. 


wire  and  the  armature  of  machine  II.,  but  not  through  its  series 
coils. 

When  a  dynamo  is  run  in  parallel  with  secondary  batteries,  the 
shunt  dynamo  only  need  be  taken  into  consideration. 

If  there  are  neither  secondary  batteries  nor  other  dynamos  it  is 
most  suitable  to  employ  the  compound  winding,  since  it  gives  a  con- 
stant voltage  as  long  as  the  armature  speed  does  not  vary. 

The  series  dynamo  is  suitable  for  supplying  current  for  a 
single  circuit  either  for  a  large  number  of  series-connected  glow- 
or  arc-lamps,  or  for  a  single  motor  as  a  generator  for  power 
transmission,  as  previously  described,  or  for  boosters  to  raise 


WORKING  OF  DYNAMOS  IN  PARALLEL 


201 


the  voltage  in  proportion  to  the  current.  In  some  cases  several 
series  dynamos  have  been  connected  in  series,  serving  as  generators 
for  a  number  of  series  motors  connected  in  series.  With  this  arrange- 
ment of  dynamos  pressures  of  some  thousands  of  volts  have  been 
produced  and  used  for  long-distance  power  transmission.  The  cases 
are,  it  must  be  added,  quite  exceptional.  For  power  transmission 
over  long  distances  alternating  currents  are  employed  almost  ex- 
clusively. 


Switching  Dynamos   in  Parallel 


When  starting  a  dynamo  which  has  to  be  run  in  parallel  with 
either  a  secondary  battery  or  another  dynamo,  we  have  to  be  quite 
certain  that  the  leads  are  of  the  right  polarity.  For  if  we  connected 
the  positive  pole  of  one  machine  with  the  negative  pole  of  another, 
and  vice  versd,  the  two  machines  would  not  be  connected  in 
parallel,  but  in  series  without  any  external  resistance,  giving  a  short 
circuit  supplied  at  double  the  voltage. 
To  make  sure  about  the  polarity, 
proceed  in  the  following  way:  Bring 
both  machines  to  the  same  voltage, 
say  110  volts,  then  close  the  switch 

1  (see   Fig.   201),   and   connect  the 
ends    of    two    series-connected    110 
volt    lamps    with    the    contacts    of 
the  switch  2,  which  must  be  kept 
open.     If   the   polarity    of   the   two 
machines    is    all    right,    no    voltage 
can  exist  between  the  poles  of  switch 
2,  and  consequently  the  lamps  cannot 
glow.     If,   on   the   other  hand,   the 
polarity    of    the    two    machines    is 
wrong,  then  there  will  be  a  double 
voltage — 220   in   the   case   supposed 

—between  the  two  contacts,  and  the  two  lamps  will  glow  with  their 
normal  intensity.  The  machines  have  then  to  be  stopped,  and  the 
cables  of  one  machine  reversed. 

Instead  of  changing  the  cable  connections  the  polarity  of  the 
machines  may  be  altered.     For  this  purpose  the  brushes  of  machine 

2  must  be  lifted  off  the  commutator,  and  switches  1  and  2  closed. 
By  doing  this  machine  II.  is  excited  in  the  right  direction.     Now 


FIG.  201.— First  Method  of 
securing  Right  Polarity. 


202 


ELECTRICAL  ENGINEERING 


Voltmeter 


open  switch  2,  and  put  the  brushes  on  the  commutator,  when  it  will 
be  found  that  the  polarity  of  the  machine  has  been  reversed. 

If  the  machines  are  provided  with  two-pole  switches  instead  of 
single-pole  ones,  the  procedure  for  finding  the  polarity  just  described 

may  be  applied  by  connect- 
ing temporarily  the  contacts 
of  one  side  of  the  switch  of 
machine  II.  by  means  of  a 
wire  or  strip  of  metal. 

To  ascertain  the  polarity 
of  a  machine,  Pole-finding 
Paper  is  sometimes  em- 
ployed. It  is  made  of  paper 
impregnated  with  a  chemical 
substance.  When  the  paper 
is  wetted  and  included  in  a 
circuit,  the  electrolytic  action 

^a^  ensues  causes  the  paper 
at  the  end  connected  with 
the  positive  pole  to  become 
of  one  colour,  whilst  around 
the  negative  end  a  different 
colour  will  be  noticed. 

The  examination  becomes 
simplest  if  there  is  a  Deprez 


/oltmeter 
switch 


FIG.  202. — Second  Method  of  securing 
Right  Polarity. 


voltmeter  provided  with  a 
voltmeter  switch  fixed  on 
If  on  closing  the  voltmeter 


the  switchboard  as  shown  in  Fig.  202. 
switch,  so  that  it  indicates  the  voltage  of  first  one  machine  and  then 
the  other,  the  pointer  of  the  voltmeter  is  deflected  in  the  same 
direction  in  both  cases,  the  machines  are  of  the  same  polarity;  if 
otherwise,  then  the  machines  are  of  opposite  polarity. 

A  similar  method  is  used  in  putting  compound  dynamos  in  parallel, 
but  after  throwing  them  together  for  the  first  time  it  must  be  found 
that  the  series  fields  act  together.  It  is  necessary  to  close  the 
equalizer  connection  switch  either  before  or  simultaneously  with 
the  other  connections.  Triple-pole  switches  are  generally  used  for 
this  purpose. 


CHAPTER  VTI 
ELECTRIC  LIGHTING 

Glow  Lamps 

ONE  of  the  first  phenomena  of  the  electric  curreat  with  which  we 
became  acquainted  was  the  heating;  of  a  wire  through  which  a 
current  flows.  The  first  idea  was,  therefore,  to  heat  metal  wires 
by  the  electric  current  to  such  a  high  degree  as  to  cause  them  to 
glow  and  emit  light.  The  common  metals,  however,  alter  their 
nature  when  heated  in  the  air,  and  therefore  a  metal  which  has  the 
property  of  not  changing  its  nature,  such  as  platinum,  must  be 
employed.  The  incandescent  or  glow  lamps,  manufactured  in  this 
way  are  very  expensive,  and,  further,  have  a  serious  defect.  Metals 
do  not  grow  bright  until  they  are  raised  to  a  temperature  which  is 
near  to  their  fusing  point.  Hence,  if  through  a  platinum  lamp  a 
current  flows  which  is  a  little  greater  than  the  normal  one,  the 
platinum  filament  will  instantly  fuse. 

Fortunately  there  is  a  solid  conductor  which  is  neither  a  metal 
nor  fusible.  This  conductor  is  carbon.  If  in  the  open  air  we  heat 
a  carbon  filament  to  such  an  extent  as  to  make  it  incandescent,  it 
will  soon  be  burnt.  Hence  the  electric  heating  of  the  filament  must 
be  done  in  the  absence  of  oxygen,  a  gas  necessary  for  combustion. 
This  has  been  effected  by  enclosing  the  filament  in  a  glass  bulb  from 
which  the  air,  and  with  it  the  oxygen  contained  in  the  air,  has  been 
carefully  exhausted. 

The  greatest  practical  difficulty  consisted  in  finding  out  a  method 
of  obtaining  carbon  strips  of  sufficiently  small  sectional  area  and 
regular  structure.  This  has  been  overcome  by  either  carbonizing  a 
cotton  or  silk  thread  directly,  or  a  filament  formed  by  "squirting" 
a  solution  of  cellulose  through  a  fine  nozzle  at  high  pressure. 
Cellulose  is  the  chief  constituent  of  such  vegetable  substances  as 
cotton,  linen,  paper,  etc. 

The  first  to  make  carbon  lamps  practical  were  Edison  and  Swan. 
The  former  used  a  carbonized  and  horseshoe-shaped  fibre  of  bamboo, 

203 


204  ELECTRICAL   ENGINEERING 

enclosed  in  a  glass  bulb  from  which  the  air  was  exhausted.  The 
connection  between  the  carbon  and  the  external  conducting  wires 
was  secured  by  short  pieces  of  platinum  wire  fused  through  the  glass 
Since  then,  the  manufacture  of  glow  lamps  has  been  very  much 
improved.  Nowadays  the  filament  is  generally  made  from  cellulose 
in  the  way  described  above. 

We  shall  now  briefly  deal  with  the  method  of  making  modern 
glow  lamps.  The  filaments  of  cellulose,  having  been  dried,  are  cut 
to  about  the  desired  length,  sufficient  margin  being  allowed  for 
making  connections  with  the  platinum  wires,  which  pass  through  the 
bulb  to  the  external  circuit.  They  are  then  subjected  to  the  process 
of  "carbonizing/'  which  converts  them  into  solid  carbon  filaments. 
Each  filament  is  then  held  by  clips  connected  with  suitable  termi- 
nals, by  means  of  which  connection  can  be  made  with  a  dynamo  or 
a  secondary  battery.  Next,  the  suspended  filament  is  placed  in  an 
atmosphere  of  gas  rich  in  carbon,  and  a  current  sufficiently  strong 
to  raise  it  to  a  white  heat  is  passed  through  the  filament.  If  there 
should  be,  as  is  generally  the  case,  any  inequality  in  the  filament, 
causing  a  variation  in  its  resistance,  one  portion  will  be  raised  to  a 
higher  temperature,  and  upon  this  hotter  section  a  greater  deposit  of 
carbon  will  take  place.  This  process  is  therefore  continued  until 
the  filament  is  of  equal  thickness,  that  is  to  say,  until  it  becomes 
uniformly  luminous  throughout. 

The  glass  in  which  the  platinum  wire  with  the  carbon  filament 
is  fixed  is  now  fused  to  the  bottom  part  of  the  bulb,  and  finally  the 
latter  is  exhausted  of  its  contained  air  and  moisture. 

For  connecting  the  filament  to  the  external  circuit  many  methods 
are  employed.  The  type  of  holder  generally  used  in  England  is 

known    as    the    Swan    or    Bayonet 
holder.      The  lamp  is,  in  this  case 
(see   Fig.    203),   provided   with   an 
insulated    brass    collar    fixed    with 
cement,    the    filament    being    con- 
nected to  the  two  brass  segments 
FIG.  203.— Brass  Cap  of  Glow  Lamp    embedded  in  the  cement.     The  collar 
(The  General  Electric  Co.).          has  two  small  side  pins,   which  fit 

into   the    "bayonet   joint"    holder. 

There  are,  besides  this,  many  other  types  of  lamp-holders,  such  as, 
for  instance,  the  Edison,  the  Siemens,  and  others. 

Generally  glow  lamps  are  tested  by  means  of  a  photometer  before 
they  are  sent  out,  and  their  candle-power,  as  well  as  the  voltage  at 
which  they  are  to  be  used,  is  marked  on  them.  If  a  lamp,  designed 
to  give  16-candle-power  at  a  voltage  of  110,  be  connected  with  a 
lower  voltage,  it  will  give  out  less  than  sixteen  candles;  if,  however,  it 
is  connected  with  a  higher  voltage,  it  will  burn  with  greater  candle- 
power.  The  use  of  lamps  on  a  higher  voltage  than  that  for  which 


ELECTRIC  LIGHTING  205 

they  are  designed  and  tested,  destroys  them  after  a  short  time.  The 
life  of  an  incandescent  lamp,  or  the  number  of  hours  that  it  can 
maintain  illumination,  varies  considerably,  but  as  an  average  period 
for  such  lamps,  which  are  used  at  the  right  voltage,  about  1000  hours 
may  be  taken.  With  lamps  that  have  been  in  use  for  some  time, 
the  vacuum  deteriorates  more  or  less,  the  carbon  of  the  filament  is 
deposited  on  the  interior  of  the  bulb,  thus  diminishing  its  trans- 
parency, till  finally  the  filament  is  broken  at  its  weakest  point,  and 
the  lamp  becomes  useless. 

If  a  lamp  is  supplied  with  a  higher  than  its  normal  voltage,  this 
reduction  of  the  luminous  effect  and  the  destruction  of  the  filament 
takes  place  much  more  rapidly. 

Tests  on  lamps  burning  on  a  higher  pressure  than  the  normal 
voltage  prove  that  their  efficiency — that  is  to  say,  the  ratio  of 
the  light  emitted  to  the  watts  absorbed  by  the  lamp — is  higher 
than  when  the  lamps  burn  at  the  proper  voltage.  The  16-candle- 
power  glow  lamps  usually  employed  consume  about  50  to  55  watts  (in 
all  the  examples  we  have  given  in  the  first  part  of  this  book  we  have 
assumed  this  consumption  to  be  55  watts  =110  volts  X  0.5  amps.), 
whereas,  lamps  burning  with  a  higher  than  their  normal  voltage 
consume  a  greater  number  of  watts,  but  the  light  emitted  by  them 
is  increased  to  a  far  greater  extent  than  their  watt  consumption. 
This  fact  has  been  taken  advantage  of  when  manufacturing  glow 
lamps  of  higher  efficiency — for  instance,  lamps  which  require  2J,  2, 
or  even  less  watts  per  candle-power.  These  lamps  deteriorate  far 
more  rapidly  than  those  having  a  lower  efficiency.  Hence  the 
advantage  of  the  lower  cost  of  current  when  employing  "  high 
efficiency"  lamps  is  diminished  by  the  necessity  of  frequent 
renewals.  The  lamps  which  are  most  generally  in  use  consume 
about  3  to  3J  watts  per  candle-power. 

Until  a  few  years  ago,  filaments  for  a  higher  voltage  than 
110  could  not  be  satisfactorily  manufactured.  For  a  higher 
voltage  the  filament  has,  naturally,  to  be  longer  and,  at  the  same 
time,  thinner.  Such  a  filament  is,  of  course,  very  fragile,  and  for 
a  long  time  the  difficulty  of  manufacture  was  insurmountable.  By 
improving  the  quality  of  the  carbon,  this  difficulty  has  been  over- 
come, and  nowadays  glow  lamps  for  220,  and  even  250  volts  are 
manufactured  almost  of  the  same  quality  as  110- volt  lamps. 

A  new  system  of  electric  incandescent  lamps  has  been  invented 
by  Professor  Nernst,  of  Goettingen.  He  employs  as  filaments 
second-class  conductors — that  is  to  say,  bodies  which  are  insulators  in 
a  cold  state,  but  become  conductors  of  the  electric  current  when 
heated  nearly  to  redness.  The  filament  has  therefore  to  be  heated 
before  the  lamp  can  be  made  to  glow.  This  may  be  effected  either 
by  means  of  a  small  spirit-lamp  or,  automatically,  by  means  of  a 
platinum  coil,  surrounding  the  filament. 

The  diagram  of  connections  for  a  lamp  in  which  the  filament  has 
to  be  heated  by  a  flame,  is  shown  in  Fig.  204.  The  automatic  type 


206 


ELECTRICAL  ENGINEERING 


is  like  Fig.  205.    Here  the  current  traversing  the  lamp  from  the  +  to 

+  -f 


:FiG.   204— Nernst  Lamp— Fila- 
ment Heated  by  Flame. 


FIG.  205. — Nernst  Lamp  with  Electric 
Heating. 

the  —  pole  has  two  ways  open  :  one 
through  the  elastic  armature  of  a  small 
electro-magnet  to  the  platinum  heating- 
coil,  and  another  one  through  the  winding 
of  the  electro-magnet,  next  through  a 
series  resistance  of  iron  wire  (marked  by 
a  zigzag  line  in  the  diagram),  and  then 
through  the  short  thick  filament  or  rod 
made  of  the  special  substance.  On  switch- 
ing in  the  lamp,  the  rod  is  still  cold,  and 
thus  not  conducting;  the  current  can 
therefore  only  flow  through  the  armature 
and  the  heating-coil.  As  soon  as  the  rod 
is  made  to  glow  by  the  heating  effect  of 
the  platinum  coil,  the  current  traverses 
the  second  path  through  the  winding  of 
the  electro-magnet,  the  series  resistance, 
FIG.  206.-Six-Glower  Outdoor  and  the  rod  itgelf .  the  electro-magnet  is, 

therefore,    able   to    attract    the  _  elastic 

armature,  and   disconnect    the  first  circuit,  the  platinum  coil  being 
then  switched  out  of  circuit. 


ELECTRIC   LIGHTING  207 

A  resistance  in  series  is  required  with  both  types  of  the  Nernst  lamp, 
since  the  rod  of  special  material  is  very  sensitive  to  variations  of  the 
current,  and  without  this  steadying  resistance  would  melt  at  the  slight- 
est rise  of  voltage.  The  steadying  resistance  is  made  of  iron  wire,  whose 
resistance  increases  comparatively  rapidly  with  rise  of  temperature. 

The  materials  from  which  the  glow-rods  are  made  stand  a  far 
higher  temperature  than  platinum  or  carbon.  The  luminosity  of  a 
source  of  light  being  greater  the  higher  the  temperature  of  the  glowing 
material,  the  Nernst  lamp  is  more  efficient  than  that  of  a  carbon  fila- 
ment glow  lamp.  It  consumes  only  about  1£  watts  per  candle-power. 

Since  the  materials  employed  for  the  Nernst  lamp  have,  even 
in  a  hot  state,  a  far  higher  specific  resistance  than  carbon,  the  glow- 
rods  are,  for  a  given  voltage  and  candle-power,  far  shorter  and  thicker 
than  the  corresponding  carbon  filaments.  Thus  the  rods  are  much 
more  solid,  and  can  be  manufactured  for  220,  300,  and  even  400  volts. 

Much  thought  has  been  given  recently  to  produce  a  material  which 
will  stand  a  higher  temperature  than  carbon  for  filaments  for  incan- 
descent lamps.  There  has  been  brought  out  a  lamp  having  a  filament 
of  titanium.  This  lamp  will  operate  with  a  life  of  1000  hours,  con- 
suming only  2  watts  per  candle  instead  of  3.1  taken  by  ordinary  in- 
candescent lamps.  A  filament  made  of  uranium  also  gives  excellent 
results.  Unfortunately,  these  metals  are  not  so  common  as  to  make 
the  lamps  cheap.  Other  metals  are  being  tried,  notably  tungsten, 
which  gives  even  better  economy  with  satisfactory  life.  When  it  is 
realized  how  little  of  the  energy  delivered  from  a  dynamo  actually 
appears  as  light,  it  can  be  understood  how  important  any  develop- 
ments of  improvements  in  incandescent  lamps  are  to  the  electrical 
industry.  Undoubtedly,  the  time  is  not  far  off  when  lights  will  con- 
sume far  less  energy  than  at  present. 

Arc  Lamps 

Whenever  a  circuit  is  broken  a  spark  is  produced.  We  must 
now  try  to  make  clear  why  this  should  be.  On  opening  a  switch 
or  disconnecting  a  live  main,  the  actual  breaking  of  the  current 
requires  a  definite  time.  During  this  time  the  contact,  originally 
a  very  good  one,  becomes  worse  and  worse,  and  the  surface  of  the 
touching  parts  becomes  smaller  and  smaller.  The  result  is  that 
resistance  is  introduced,  and  heat  is  produced.  The  temperature 
becomes  finally  very  great,  so  that  the  ends  of  the  conductors 
begin  to  glow,  and  emit  glowing  metal  vapour,  which,  even  after 
some  time,  when  the  two  conductors  have  been  separated  a  little 
distance  from  each  other,  may  cross  the  gap,  and  form  a  conducting 
luminous  bridge,  called  the  arc. 

To  obtain  a  continuous  arc,  metal  rods  are  not  suitable,  because 
they  soon  fuse  and  evaporate.  Carbons  are  in  every  way  preferable. 
The  carbons  which  are  connected  with  the  two  mains  are  first  of  all 
brought  together,  so  that  the  current  can  flow  from  one  carbon  to 


208  ELECTRICAL  ENGINEERING 

the  other.  The  contact  surface  offers  a  comparatively  high 
resistance,  so  that  the  carbon  ends  begin  to  glow.  Then  they  are 
removed  some  sixteenths  of  an  inch  from  each  other.  The  arc 
that  is  formed  continues,  since  the  highly  heated  air  and  the  carbon 
vapour  form  between  the  two  electrodes  a  conductor  of  very  high 
resistance.  The  arc  itself  does  not  emit  the  greatest  part  of  the 
light,  but  the  carbon  points,  especially  the  positive,  are  the  chief 
source  of  light. 

The  two  carbons  are  not,  with  a  continuous  current,  consumed 
at  an  equal  rate,  the  consumption  of  the  rod  connected  to  the 
positive  pole  of  the  dynamo  being  approximately  twice  as  fast  as 
that  of  the  other,  or  negative  carbon.  After  burning  some  time, 
the  end  of  the  positive  rod  becomes  concave,  forming  a  crater,  and 
in  the  hollow  of  this  crater  the  most  intense  heat  is  developed, 
making  it,  therefore,  the  chief  source  of  light.  The  negative  rod 
is  gradually  consumed  until  its  extremity  is  of  a  conical  shape. 
With  continuous-current  lamps  the  lower  or  negative  carbon  is 
usually  thinner  than  the  upper  one,  the  object  being  to  make  the 
consumption  of  the  carbons  equal  as  regards  length. 

If  we  calculate  the  current  strength  of  an  arc  lamp  after  Ohm's 
Law,  we  arrive  at  an  incorrect  result,  just  as  with  the  calculation 
of  the  current  in  a  liquid  (see  p.  179).  The  arc  is,  like  a  storage 
cell,  the  seat  of  a  back  E.M.F.,  but  which  ceases  immediately  the 
current  stops.  The  back  E.M.F.  of  the  arc  is  very  considerable, 
and  the  current  has  also  to  overcome  the  ohmic  resistance  of  the 
arc.  Hence  the  applied  pressure  must  be  above  a  certain  value. 
The  voltage  required  for  an  ordinary  arc  is  about  35  to  40  volte. 
With  special  types,  where  the  arc  is  formed  in  a  partial  vacuum, 
and  is  very  long,  the  voltage  may  be  80  or  more.  It  is  quite 
impossible  to  get  a  continuous  arc  with  a  voltage  of  less  than  30  to  35. 
Glow  lamps  may,  as  we  know,  be  built  for  any  pressure,  since  they 
have  an  ohmic  resistance  only,  and  thus  the  dimensions  of  the 
filament  may  be  made  according  to  the  voltage.  There  are,  for 
instance,  glow  lamps  which  require  a  voltage  of  but  2,  and  can 
therefore  be  fed  by  a  single  accumulator  cell.  With  arc  lamps 
this  is  impossible. 

For  an  arc  lamp  a  special  mechanism  is  necessary.  First  of 
all,  the  two  carbon  rods  have  to  be  placed  in  contact,  and  then 
have  to  be  separated, .  so  that  an  arc  is  formed  between  them. 
Further,  in  order  to  maintain  the  arc,  it  is  also  essential  that 
some  device  should  be  provided  for  "  feeding ''  the  carbons 
together  at  a  rate  proportionate  to  their  consumption.  Generally 
the  electro-magnetic  effect  of  the  current  is  used  to  operate  this 
mechanism. 

In  Fig.  207  is  shown  one  of  the  different  types.  The  upper  and 
lower  carbon  holders  are  suspended  from  a  flexible  wire  or  a  chain, 


ELECTRIC  LIGHTING 


209 


passing  over  a  roller.  The  upper  carbon  holder  is  provided  with 
an  iron  core,  which  moves  within  a  fixed  coil.  The  latter  is  con- 
nected in  series  with  the  arc,  so  that  the  current  forming  the  arc 
also  flows  through  this  coil.  If,  now,  the  iron  core  is  pulled  up 
by  the  action  of  the  coil,  the  upper  carbon  holder  is  lifted,  and 
the  bottom  carbon  is  lowered,  so  that  the  carbons  are  separated. 
If,  on  the  other  hand,  the  iron  core  is  less  attracted  by  the  coil,  it 
will  descend,  causing  the  carbons  to  approach. 
By  selecting  the  weight  of  the  iron  core,  then, 
at  a  certain  current  strength,  the  attraction 
of  the  solenoid  and  the  weight  of  the  core 
are  just  balanced.  Assuming  now  the  arc 
lamp,  having  a  resistance  in  series,  to  be  con- 
nected to  a  constant  voltage,  then,  after  a 
short  time,  due  to  the  burning,  the  resistance 
of  the  arc  will  be  increased,  and  the  current 
will  be  decreased.  With  the  weakened 
current  the  attractive  power  of  the  coil 
becomes  smaller,  the  weight  of  the  carbon 
and  its  holder,  therefore,  causes  the  carbons 
to  be  brought  nearer  together,  until  the 
diminution  of  the  arc  resistance  causes  the 
current  to  be  increased  to  such  an  extent 
as  to  again  balance  the  weight  of  the  carbon 
holder.  If,  on  the  other  hand,  the  iron  core 
falls  too  far  down,  so  that  the  arc  is  shorter 
than  normal,  then,  the  resistance  being  de- 
creased, the  current  becomes  greater,  and  the 
attractive  force  of  the  solenoid  overcomes 
the  weight  of  the  iron  core,  with  the  result 
that  the  carbons  are  separated  a  little. 

The  regulating  solenoid  being  connected  in  series  with  the  arc, 
this  lamp  is  called  a  series  lamp.  It  tends,  as  we  have  seen,  to 
maintain  a  constant  current,  and  is  very  suitable  for  use  on  a 
constant  voltage  supply. 

Let  us  now  arrange  a  number  of  such  lamps  in  series,  and  in 
a  circuit  in  which  the  current  strength  is  kept  constant  by  any 
means,  then  it  will  be  found  that  the  regulating  mechanism  is  abso- 
lutely useless.  For  it  is  evident  that  as  long  as  the  current  in  the 
circuit  remains  constant,  the  lamp  will  not  regulate — even  if,  owing 
to  the  consumption  of  the  carbon,  the  arc  much  exceeds  its  normal 
length.  Hence  the  voltage  at  the  terminals  of  the  lamp,  which  is 
usually  40  to  45,  may  grow  to  80,  and  even  more.  If,  finally,  the 
resistance  of  the  lamp  becomes  so  great  that  the  dynamo  is  unable 
to  supply  this  higher  voltage  at  the  normal  current,  then  the  latter 
will  decrease,  thus  causing  all  the  solenoids  to  affect  the  length  of 


FIG.  207  — Series  Arc 
Lamp. 


210 


ELECTRICAL  ENGINEERING 


the    arcs,  although  in  some  lamps  the  length  of  the  arc  might  have 
been  the  right  one. 

In  such  cases,  instead  of  series  lamps,  shunt  lamps  may  be 
employed.  A  scheme  of  a  shunt  lamp  is  shown  in  Fig.  208.  The 
solenoid,  consisting  of  many  turn^  of  a  very  fine  wire,  is  arranged 
so  as  to  tend  to  lift  the  lower  carbon  holder.  The  solenoid 
itself  is  connected  with  the  two  terminals  of  the  lamp,  thus  being 
in  shunt  with  the  arc;  therefore  the  current  traversing  the 
solenoid  is  greater  the  higher  the  voltage  of  the  arc.  At  the 
proper  voltage  of  the  arc;  the  weight  of  the  iron  core  is  just  counter- 
balanced by  the  attraction  of  the  solenoid.  But  if,  due  to  a  burning 
of  the  carbons,  the  length  of  the  arc  is  increased,  its  voltage  will  also 
rise,  provided  that  the  current  strength  remains  constant.  Thus,  if, 
owing  to  the  higher  voltage,  a  larger  current  flows  through  the  shunt 


FIG.  208. — Shunt  Arc  Lamp. 


FIG.  209. — Differential  Arc  Lamp. 


coil,  the  action  of  the  solenoid  will  preponderate,  lift  the  bottom,  and 
lower  the  upper  carbon  holder,  so  that  the  arc  is  again  shortened  to 
its  right  length.  This  lamp  therefore  tends  to  maintain  constant 
voltage. 

A  third  kind  of  an  arc  lamp  is  the  differential  lamp,  a  scheme 
of  which  is  shown  in  Fig.  209.  In  this  lamp  we  have  two  coils, 
which  act  against  one  another.  One  of  these  coils  is  a  series  coil, 
tending  to  lift  the  upper  carbon  holder,  and  to  lengthen  the  arc;  the 
other  a  shunt  coil,  tending  to  lift  the  bottom  carbon  holder,  and 
hence  to  shorten  the  arc. 


ELECTRIC  LIGHTING 


211 


This  lamp  will  therefore  combine  the  properties  of  the  other  two 
types,  and  be  suitable  for  use  on 
both  constant  current  and  constant 
voltage  circuits. 

An  essential  requirement  of  an 
arc  lamp  is  a  means  of  damping 
the  regulating  mechanism,  so  as  to 
prevent  any  sudden  or  violent 
movement  of  the  carbons,  causing 
a  flickering  of  the  light.  Several 
devices  have  been  arranged  for 
causing  the  mechanism  to  act  in 
a  gradual  manner.  One  of  the 
most  usual  consists  of  a  roller, 
over  which  passes  a  flexible  cord 
that  carries  the  carbon  holder. 
This  roller  drives  through  several 
toothed  wheels  a  fan.  The  latter 
is  made  to  rotate  at  a  great  speed 
against  the  resistance  of  the  air. 
Any  sudden  increase  of  the  speed 
of  the  roller  is  prevented,  owing 
to  the  great  resistance  that  the  air 
offers  to  the  increase  of  speed  of 
the  fan. 

Another  damping  arrangement 
consists  of  a  piston,  moving  within 
a  cylinder,  with  but  little  play,  so 
that  the  enclosed  air  is  compressed 
or  expanded,  preventing  sudden 
motion  of  the  piston. 

Fig.  210  shows  the  general 
arrangement  of  the  parts  of  an  arc 
lamp,  and  Fig.  211  shows  the 
principle  of  a  Kfizik  or  Pilsen 
differential  lamp.  To  the  explana- 
tion given  with  the  general  scheme 
of  a  differential  lamp,  we  h,°ve  to 
add  the  following  for  the  Knzik 
lamp : — The  iron  cores,  a  and  6,  are, 
as  may  be  seen  from  the  dotted 
lines  in  the  figure,  not  cylindrical, 
but  conical.  They  are  enclosed 
within  brass  tubes,  and  by  small 
guiding  rollers  a  true  vertical 


FIG.   210. — Arc    Lamp     (The  Elec- 
trical Company). 


motion  is  ensured.     Over  the  iron  cores  the  series  coil  g  and  the 


212 


ELECTRICAL  ENGINEERING 


shunt  coil  /  are  respectively  wound.    When  the  lamp  has  been  freshly 

trimmed  and  the  carbons  are  long,  the  core 
of  the  upper  carbon  holder  at  its  highest, 
and  that  of  the  lower  carbon  is  at  its  lowest 
position,  whereas  at  the  end  of  the  burning 
hours  the  opposite  would  be  the  case. 
With  cores  of  a  cylindrical  shape,  the 
attractive  power  of  the  coil  on  the  core 
would  vary  according  to  the  position  of  the 
core  to  the  coil.  On  the  other  hand,  the 
conical  shape  of  the  cores  ensures  that 
the  attractive  forces  will  at  these  and  other 
positions  be  balanced,  provided  that  through 
both  the  series  and  the  shunt  coil  the  normal 
current  is  flowing. 

The  roller  c,  over  which  passes  the  cord 
bearing  the  carbon  holders,  is  provided  on 
its  circumference  with  fine  teeth.  Into  the 
latter  a  ratchet  h  interlocks,  which  is  not 
only  movable  about  its  axis,  but,  being 
fixed  within  an  oval  hole,  is  also  movable 
for  a  short  distance  upwards.  When  the 
lamp  is  not  in  circuit,  the  two  carbons 
touch  each  other,  and  the  shunt  coil  is 
short-circuited.  On  closing  the  switch, 
the  mains  being  connected  to  the  terminals 
marked  +  and  — ,  then  the  series  coil  moves  its  core  6  upwards.  This 
upward  motion  can,  however,  only  take  place  so  far  as  is  allowed  by 
the  length  of  the  oval  hole.  This  length  is  selected  so  as  to  give  the 
right  length  of  the  arc.  The  latter  then  has  its  normal  voltage,  and 
the  series  and  shunt  coil  are  therefore  counterbalanced.  As  the 
lamp  continues  in  use,  the  shunt  coil  /  is  able,  without  any  im- 
pediment from  the  ratchet,  to  lift  the  lower  carbon  holder  and 
shorten  the  arc,  since  this  ratchet  is  arranged  so  as  to  stop  a  reverse 
motion. 

There  are  numerous  other  types  of  arc  lamps,  which  the  limits  of 
this  book  preclude  us  from  describing. 

It  is  of  great  importance  to  use  a  resistance  in  series  with  an  arc 
lamp.  If  we  connected  an  arc  lamp  directly  on  to  a  40-volt  circuit, 
then  the  variations  of  the  current  would  be  excessive,  and  quite 
beyond  the  power  of  the  regulating  mechanism  to  control.  On 
switching  on  an  arc  lamp,  the  carbons  are  brought  to  directly  touch 
each  other,  whilst  the  lamp  does  not  yet  produce  any  back  E.M.F. 
Hence  the  lamp  resistance  is  small,  and  the  current  therefore  excessive. 
Any,  even  the  smallest,  lengthening  or  shortening  of  the  arc  would 
produce  a  great  variation  of  the  current,  since  any  change  of  length 


FIG.  211.— The  Kfizik  Arc 
Lamp. 


ELECTRIC  LIGHTING 


213 


FIG.  212. — Arc  Lamp  Resistance  without  cover. 
(The  Electrical  Company}. 


of  the  arc  is  followed  by  an  increase  or  decrease  of  the  back  E.M.F. 
Assuming,  for  example,  the  back  E.M.F.  to  be  39  volts  in  one,  and 
36  volts  in  another  case,  then  the  difference  between  the  terminal 
voltage  and  back  E.M.F.  will  be  1  and  4  volts  respectively.  The 
ohmic  resistance  of  the 
lamp  remaining  the 
same,  the  current  will 
be  four  times  as  much 
in  the  second  as  in  the 
first  case.  In  the  feed- 
ing of  arc  lamps  from  a 
dynamo,  the  machine 
voltage  is  therefore  al- 
ways made  larger  than 
the  lamp  voltage  should 
be,  and  a  constant  resistance  is  kept  in  series  (see  Figs.  212  and  213), 
which  absorbs  the  superfluous  voltage.  The  line  voltage  is  then 
best  selected  about  60  to  65,  so  that  in  the  resistance  20  to  25 
volts  are  absorbed.  If,  now,  by  any  change  of  the  length  of  the  ^re 
its  voltage  be  varied, 
say  from  39  to  36,  this 
will  cause  only  an  un- 
important rise  of  cur- 
rent, because  in  the  first 
case  the  difference  be- 
tween the  electro-motive 
forces  will  be  65  -  39 
=  26  volts,  and  in  the 
second  case  65  —  36  = 
29  volts.  Hence,  if  the 
total  resistance  be  3a>, 
the  current  would  be  8.6  amps,  in  the  first,  and  9.6  amps,  in  the 
second  case,  the  difference  between  these  two  currents  being  only 
1  amp.  Further,  when  the  lamp  is  first  connected  with  the  current 
supply,  and  the  carbons  are  actually  touching,  nevertheless  the 
current  cannot  become  too  large.  Its  maximum  value,  of  course 
only  for  a  brief  period,  will  be  65  volts  -r-  3a>  =  21.6  amps. 

The  larger  the  series  resistance  the  steadier  the  lamp  will 
burn.  On  the  other  hand,  the  resistance  wastes  electrical  energy; 
hence,  for  economical  working,  the  series  resistance  should  be 
made  as  small  as  possible,  i.e.  just  as  small  as  will  ensure  good 
regulation. 

If  on  110-volt  mains  single  lamps  are  used,  then  about  70  volts 
are  absorbed  by  the  resistance,  i.e.  about  two-thirds  of  the  total 
energy  is  rendered  useless.  Hence,  with  110-volt  mains,  two  lamps 
are  often  used  in  series,  with  a  resistance  absorbing  about  30  volts; 


FIG.  213. — Arc  Lamp  Resistance  enclosed. 
(The  Electrical  Company). 


214 


ELECTRICAL   ENGINEERING 


with  150-volt  mains,  the  lamps  are  switched  in  groups  of  three  in 

series  with  a  resistance;  and  with 
220-volt,  generally  in  groups  of  four. 
There  are  also  special  connections, 
where  groups  of  three  smaller  arc 
lamps  are  run  on  110-  or  120-volt 
mains  without  a  permanent  resistance, 
but  merely  with  a  starting  resistance. 
In  these  cases  special  precautions  have 
to  be  made;  nevertheless,  the  lamps 
can  never  burn  as  satisfactorily  as  in 
groups  of  two. 

Where  the  lighting  is  exclusively 
by  means  of  arc  lamps,  and  the  use  of 
glow  lamps  need  not  be  considered, 
connection  of  the  lamps  in  series  is  a 
frequent  method.  In  this  case  there 
is  only  one  circuit,  in  which  all  the 
arc  lamps — for  example,  10,  20,  or 
30 — are  connected  in  series.  The 
voltage  of  the  dynamo,  or  arc  lighter, 
has  therefore  to  be  high;  viz.,  if  for 
one  lamp  with  its  regulating  resistance 
we  assume  a  voltage  of  50,  then  30 
lamps  will  need  1500  volts.  Since 
the  extinguishing  of  one  lamp  would 
prevent  current  supply  to  the  re- 
mainder, there  must  be  provided  an 
alternative  path  in  each  lamp,  which 
either  short-circuits  it,  or  inserts  a 
compensating  resistance.  In  this  sys- 
tem the  current  of  the  machine,  gener- 
ally provided  by  a  series  dynamo,  has 
to  be  kept  constant,  and  its  voltage 
must  be  varied  according  to  the 
number  of  lamps  running.  This 
method  is  not  now  so  commonly  in 
use  as  it  once  was. 

With  arc  lamps  a  more  economical 
lighting  is  effected  than  with  glow 
lamps.  An  arc  lamp  burning  with  10 
amps,  has  a  luminous  power  of  about 
500  to  1000  candles.  Since  (including 

the  resistance)  an  arc  lamp  requires  about  55  volts,  we  get  for  an 
electrical  power  of  550  watts  500  to  1000  candles,  from  which 
it  follows  that  we  have  to  spend  only  i  to  1  watt  per  candle-power, 


FIG.  214. — Enclosed  Arc  Lamp. 
(The  Electrical  Company'}. 


ELECTRIC  LIGHTING 


215 


whereas  a  glow  lamp  requires  3  to  3J  watts  per  candle.  Arc  lamps 
are  also  constructed  for  8,  6,  4,  and  2  amps.,  sometimes  for  even  less. 
Smaller  arc  lamps,  however,  are  not  economical.  Further,  since  arc 
lamps  require  far  more  attendance  than  glow  lamps,  small  arc  lamps 
are  seldom  used.  On  the  other  hand,  for  the  lighting  of  streets, 
squares,  shops,  etc.,  where  much  light  is  required,  the  use  of  large 
arc  lamps  is  common. 


FIG.  215.— Magnetite  Arc  Lamp. 

A  special  application  of  the  arc  lamp  is  as  a  search  light.  Very 
large  arc  lamps  are  used  for  this  purpose,  and  the  light  is  reflected  by 
means  of  parabolic  mirrors,  so  that  it  can  be  directed  on  any  object. 

With  the  arc  lamps  hitherto  considered,  the  arc  is  formed  in  the 


216  ELECTRICAL  ENGINEERING 

air,  although  for  softening  the  exceedingly  intense  light  glass  globes 
are  always  used.  Now  there  exists  another  kind  of  arc  lamp,  whica 
burns  with  the  arc  enclosed.  Over  the  carbons  there  is  a  small  glass 
cylinder,  so  arranged  that  it  fits  round  the  carbons,  making  a  small 
and  nearly  air-tight  chamber  (see  Fig.  214).  This  cylinder  is,  of 
course,  first  filled  with  air,  but,  on  burning  for  a  short  time,  all  the 
oxygen  contained  in  the  air  in  the  small  cylinder  is  consumed.  Hence 
the  carbons  are  consumed  far  less  if  burning  in  this  enclosed  manner, 
and  these  lamps  may  be  manufactured  to  burn  a  hundred  hours  and 
longer,  whereas  the  carbons  of  common  arc  lamps  have  generally  a 
burning  time  of  only  five  to  ten  hours. 

With  "  enclosed  arc  lamps  "  the  length  of  the  arc  is  generally 
y  to  £",  so  that  the  voltage  of  the  arc  is  equal  to  about  80.  These 
lamps  may  therefore  be  connected  singly,  and  with  only  a  small  series 
resistance,  to  100-  or  110- volt  mains. 

NEW  TYPES  OF  LAMPS. 

Dr.  Auer,  of  Vienna,  employs  as  a  filament  for  a  glow-lamp  osmium,  a  material 
which  conducts  when  cold,  and  therefore  does  not  require  any  preliminary  heat- 
ing. The  efficiency  of  these  lamps  is  said  to  be  equal  to  that  of  the  Nernst  lamps. 
On  the  other  hand,  owing  to  the  low  specific  resistance  of  osmium,  they  can  best 
be  used  for  voltages  from  20  to  50. 

In  the  Bremer  arc  lamp  the  carbons  used  have  certain  salts  added  to  them, 
with  the  effect  of  increasing  the  light,  and,  at  the  same  time,  making  a  great 
change  in  its  colour. 

The  Cooper-Hewitt  lamp  consists  of  a  long  tube,  in  which  mercury  vapour  is 
heated  by  an  electrical  current.  It  requires  about  half  a  watt  per  candle-power. 
The  light  is  especially  rich  in  blue  and  violet  rays. 

The  General  Electric  Company  is  now  producing  an  arc  lamp  which  has  one 
terminal  of  metal  and  the  other  a  rod  of  magnetite.  This  gives  a  light  quite 
similar  to  the  ordinary  carbon,  but  with  the  same  energy  gives  60%  more  light. 
It  burns  150  hours,  and  does  not  have  a  glass  enclosure  around  the  arc  with  its 
attendant  cleaning  and  breakage.  This  lamp  has  recently  been  placed  upon  the 
market.  FIG.  215  is  an  illustration  of  this  lamp. 


CHAPTER  VIII 
ALTERNATING   CURRENTS 


Properties  of  Angles  Concerned  with  Alternating 

Currents 

ONE  straight  line  intersecting  another  makes  with  it  an  angle. 
Examine  Fig.  216. 

The  line  a-o  intersecting  the  line  b-o  at  o  makes  with  it  the  angle 
a-o-b]  with  c-o,  the  angle  a-o-c  etc.  Take  any  given  angle,  a-o-6, 
and  from  the  point  b  drop  a  line 
perpendicular  to  the  line  a-o,  strik- 
ing it  at  F ;  then  b-F-o  is  a  right 
angle.  Take  any  point  on  the  line 
o-b,  say  gr,  and  drop  a  perpendic- 
ular line  g-h  to  o-a.  It  can  be 
easily  shown  by  geometry  that 
the  ratio  of  the  line  b-F  to  o-F  is 
the  same  as  g-h  to  o-h,  or  the  same 
as  any  perpendicular  to  the  base 
line.  Also  the  ratio  of  b-F  to  b-o 
is  the  same  as  g-h  to  g-o,  or  the 
same  as  any  vertical  to  the  amount 
of  the  diagonal  cut  off  by  its  inter- 
section with  the  vertical.  This  ratio  of  the  vertical  to  the  per- 
pendicular is  in  Fig.  216  -j—  or  -— ,  and  is  called  the  sine  of  the 

angle  o-o-F.  Every  angle  has  a  definite  sine.  Thus,  the  sine  of  30 
degrees  equals  J. 

The  ratio  of  the  horizontal  part  cut  off  by  the  perpendicular  from 
the  diagonal  to  the  opposite  side,  that  is,  the  ratio  in  the  triangle 
o-F— 6,  of  o-F  to  o-b  is  called  the  cosine.  Every  angle  has  a  specific 

value  of  the  cosine.    For  30  degrees  it  is  -=— .     Thus,  the  sine  of 

an  angle  in  a  right-angle  triangle  is  the  ratio  of  the  side  opposite  to 
the  longest  side.  The  cosine,  the  ratio  of  the  side  adjacent  to  the 
longest  side.  Consider  in  Fig.  216  the  line  o-d  to  be  an  edgewise 
view  of  a  coil  revolving  about  o.  Let  l-l-l  represent  lines  of  force 

317 


<•'  h 


FIG.  216.—  Properties  of  Angles. 


218 


ELECTRICAL  ENGINEERING 


flowing  through  this  coil.  At  the  position  o-d  the  coil  contains  the 
maximum  number  of  lines  of  force.  As  the  coil  turns  less  and  less 
lines  of  force  go  through  the  coil  until,  when  it  reaches  the  position  o-a, 
no  lines  of  force  go  through  the  coil,  being  then  edgewise  to  them. 
The  amount  of  area  presented  to  the  lines  of  force  is  represented  by 
the  vertical  lines  c-e,  6-F,  at  the  positions  c  and  6. 

Let  the  length  of  the  line  o-d  equal  R;  then,  since  the  sine  of  the 

** p 

angle  c-o-e  equals  ^-,  c-e  equals  R  sine  c-o-e.      Also  b~F  equals  R 

sine  6-o-F.  Thus,  at  any  angle  movement  from  the  zero  reference 
line  o-a,  the  amount  of  lines  of  force  are  equal  to  RXsine  of  the 
angle  away.  At  o-a  the  angle  is  o.  Sine  0°  equals  o,  and  the  lines 
of  force  equal  R  +  o  equals  o,  or  no  lines  go  through,  which  is  evident, 
since  the  coil  is  edgewise  to  the  lines.  At  o-d  the  angle  from  o-a  is 
90  degrees.  The  sine  of  90  degrees  equals  1,  since  at  90  degrees  the 
perpendicular  and  the  diagonal  coincide  and  become  one.  If  a  curve 
be  plotted  giving  for  360  degrees  at  each  angular  position  from  0°  the 
value  of  the  flux  going  through  a  coil,  it  will  be  as  shown  in  Fig.  216, 
or,  what  is  the  same  thing,  for  all  the  values  of  the  sine.  The  curve 
will  look  like  Fig.  217,  having  a  maximum  value  at  d.  This  curve 


k 
FIG.  217. — Sine  curve  of  E.M.F.  and  Current. 

is  called  a  sine  curve,  and  represents  how  the  flux  varies  in  a  coil 
revolved  in  a  uniform  field  of  flux.  Suppose  the  coil  to  be  connected 
to  collector  rings,  as  shown  in  Figs.  64,  65, 66.  Since  in  the  coil  the 
flux  is  changing,  as  shown  in  Fig.  216,  a  voltage  must  be  generated , 
since  voltage  is  produced  by  a  change  in  the  number  of  lines  of  force 
in  a  circuit,  as  has  been  shown.  At  d,  Fig.  217,  the  flux  is  not  changing 
at  all.  Hence,  here  the  voltage  is  0,  as  shown  by  the  dotted  curve 
h-l-j-k.  At  b,  the  flux  is  having  a  maximum  rate  of  change;  hence 
the  voltage  is  a  maximum,  as  shown  at  I.  If  at  each  point  of  the 
curve  b-d-a-f-Qj  the  rate  of  change  of  flux  be  found,  the  corresponding 
voltage  can  be  determined,  as  shown  in  the  dotted  curve  h-l-j-k. 


ALTERNATING   CURRENTS  219 

This  curve  is  found  to  have  the  same  shape  as  the  other  curve.  Hence, 
the  rate  of  change  of  one  curve  gives  another  curve  of  the  same  shape. 
This  dotted  curve  is  the  voltage  curve  of  an  alternator.  It  is  a  sine 
curve.  This  curve  has  certain  important  characteristics.  If  the 
square  root  of  the  average  of  the  squares  of  all  the  vertical  lines  l-m, 
n-o,  c-d,  etc.,  be  taken,  the  result  equals  the  maximum  value  c-d+^/2 

c-d 
equals  --  equals  .707  c-d.     This  value  is  called  the  square  root  of 

the  mean  square  of  all  the  values  of  the  sine  curve.  The  plain  average 
of  all  the  verticals  equals  .637  c-d.  Thus,  the  average  value  divided 


by  the  square  root  of  mean  square  value  equals  '-^-=-  equals  .90. 

To  return  now  to  the  alternator  which  with  a  single  coil  produces 
the  voltage  as  shown  in  the  curve  called  a  sine  curve;  to  determine 
the  formula  for  the  voltage  we  have  shown  previously  that  the  average 

E.M.F.  of  a  coil  revolving  in  a  uniform  field  equals  ^       ,  where 

1UU,UUU,UUU 

N  equals  the  number  of  revolutions  of  armature  (or  coil)  per  second, 
and  (/)  equals  the  number  of  lines  of  force  threading  through  the 
coil.  But  in  a  sine  curve  the  maximum  value  equals  the  average 

value  multiplied  by  |-  when  n  equals  3.14159,  or  the  maximum  volt- 

4Ncf>        X^TT  27rX<i 

age  of  a  sme  curve  of  E.M.F.  equals  1oo,ooo,000X2  °  100,000,000" 
Also  in  a  sine  curve  the  square  root  of  mean  square  value  equals 
the  maximum  value  divided  by  \/2.  Hence,  the  square  root  of 
mean  square  value  of  the  E.M.F.  of  an  alternator  with  an  armature 

having  one  coil  and  with  two  poles  equals  ^        -f-  \/2    equals 

1  UU  ,  UUu  ;  UUU 

'  ^  .  If  there  are  n  coils  in  series,  the  value  is  n  times  as 
lUUjUUUjUuU 

4.44Nn<£ 
much'  or  100,000,000- 

The  revolutions  per  second  of  a  2-pole  dynamo,  or  the  equivalent 
of  a  dynamo  with  more  than  two  poles,  are  called  the  cycles  of  the 
dynamo,  or  of  the  circuit  which  the  dynamo  is  feeding.  If  the  speed 

per  minute  of  a  2-pole  alternator  equals  M,  the  cycles  equal  ^».     If 

•  OU 

NX2 
the  speed  of  a  4-pole  alternator  equals  N,  the  cycles  equal  . 

Thus,  the  cycles  of  a  P-pole  alternator  running  at  N  revolutions  per 

P     N 
minute  equal  ~o^^7r- 

All  electrical  measuring  instruments  record  on  their  scale  the 


220  ELECTRICAL  ENGINEERING 

square  root  of  mean  square  values.  Hence,  when  the  voltage  of  an 
alternator  is  read  on  an  instrument  the  square  root  of  mean  square 
value  is  read.  From  this  thejnaximum  value  can,  of  course,  be 
calculated  by  multiplying  by  X/2*.  Since  the  voltage  of  an  alternator 
varies  as  shown  in  Fig.  217,  and  since  the  current  is  always  propor- 
tional to  the  voltage,  the  current  curve  is  the  same  as  the  voltage 
curve,  having  its  square  root  of  mean  square  value,  etc.,  just  as  the 
voltage  curve.  Since  heat  from  an  electric  current  is  proportional 
to  the  square  of  the  current,  the  square  root  of  the  mean  square 
value  of  current  (called  sometimes  the  effective  or  virtual  current), 
when  squared  and  multiplied  by  the  resistance  through  which  it  flows, 
gives  the  same  result  as  a  direct  current  equal  to  the  square  root  of 
mean  square  current  when  squared  and  multiplied  by  the  resistance 
through  which  it  flows.  Hence,  in  calibrating  A.  C.  instruments, 
direct  current  may  be  used,  each  value  of  direct  current  equalling 
in  its  effects  on  the  instrument  the  square  root  of  mean  square,  or 
virtual,  A.  C.  current.  Referring  once  more  to  Fig.  217,  if  the  line 
b-g  be  divided  into  360  parts,  representing  360  degrees,  or  one  com- 
plete revolution,  which  gives  a  complete,  cycle,  as  shown,  the  number 


C 

FIG.  218.— E.M.F.  and  Current  90°  apart  in  Phase 


of  degrees  from  a  reference  point  b  for  any  part  of  the  curve  is  called 
its  phase.  Thus,  the  phase  of  the  point  m  is  b-l;  of  the  point  o,  o-n. 
Thus,  the  point  o  differs  in  phase  from  the  point  m  by  the  degrees 
represented  by  l-n.  Thus,  the  plus  maximum  of  the  full  curve,  that 
is,  c-d,  differs  in  phase  from  the  plus  maximum  of  the  dotted  curve 
by  90  degrees;  that  is,  the  distance  a-c.  Thus,  the  rate  of  change  of 
flux  is  90  degrees  lagging  behind  the  flux  itself.  The  sine  curves, 
therefore,  must  be  plotted  90  degrees  apart,  if  one  represents  flux 
and  the  other  the  E.M.F.  from  that  flux,  the  E.M.F.  being  later  than 
the  flux.  A  convenient  method  of  showing  sine  curves  is  to  represent 
them  by  their  maximums  or  square  root  of  mean  square  values. 
Thus,  the  two  sine  curves  shown  in  Fig.  217  can  be  represented  as 
shown  in  Fig.  218.  Here  c-d  represents  the  maximum  value,  or  square 


ALTERNATING  CURRENTS  221 

root  of  mean  square  preferably  (either  can  be  used,  since  one  equals 
V  2  times  the  other)  of  the  flux,  as  shown  in  full  line  in  Fig.  217,  and 
c-j  shows  the  maximum  value  of  E.M.F.  resulting  from  this  flux, 
differing  in  phase  by  90  degrees  as  shown.  This  method  of  plotting 
alternating  E.M.F. 's  and  currents  is  that  generally  used  by  electri- 
cians and  gives  an  eye  picture  of  the  relations  of  alternating  values. 
It  is  called  the  Vector  Diagram  method. 


Experiments  with  Alternating  Currents 

THE  first  electrical  machine  with  which  we  became  acquainted  pro- 
duced alternating  currents.  On  providing  the  Siemens  H  armature 
with  slip-rings,  and  rotating  it  in  a  magnetic  field,  we  were  enabled 
to  collect  currents  of  an  alternating  kind.  We  then  dealt  with 
devices  for  commutating  the  alternating  current,  which  originally 
is  produced  in  any  dynamo,  into  continuous  current.  We  shall 
now  consider  the  properties  of  the  unrectified  alternating  current,, 
and  the  special  types  of  machines  designed  as  alternating  current 
dynamos. 

First  of  all,  let  us  try  experiments  similar  to  those  we  made  with 
a  continuous  current.  Connect  a  wire  resistance  with  the  terminals 
of  an  alternating  current  generator.  On  turning  the  armature  the 
wire  is  heated,  and,  if  the  current  be  sufficiently  strong,  the  wire  may 
glow,  and  even  melt.  In  like  manner  an  alternating  current  will 
cause  an  incandescent  lamp  to  glow.  We  see,  therefore,  that  the 
heating  effects  of  an  alternating  current  are  like  those  of  a 
continuous  current.  This  is  easily  understood.  The  heating  of  a 
conductor,  traversed  by  an  electric  current,  does  not  depend  on  the 
direction,  but  merely  on  the  strength  of  the  current,  and  continues 
therefore  even  if  the  current  is  continuously  altering  its  direction. 

On  rotating  the  armature  of  our  two-pole  dynamo  with  a  speed 
of  about  3000  revolutions  per  minute,  thus  getting  6000  alternations 
per  minute,  or  100  per  second,  we  observe  a  perfectly  constant 
illumination  of  the  lamp.  If,  however,  we  turn  the  machine  with 
only  the  fourth  part  of  this  speed,  and  connect  with  it  a  lamp  for 
a  correspondingly  lower  voltage,  we  observe  that  the  lamp  is  not 
giving  a  constant  light,  but  flickers  like  a  gas  light  supplied  with 
gas  at  a  fluctuating  pressure.  This  phenomenon  is  readily  under- 
stood. We  know  from  our  observations  on  page  69,  that  the 
strength  of  an  alternating  current  increases  gradually  from  zero  to 
its  maximum  value,  then  decreases  to  zero,  and,  changing  its 
direction,  again  reaches  its  maximum  value,  etc.,  as  shown  in  Fig.  67. 


222  ELECTRICAL  ENGINEERING 

Thus  the  carbon  filament,  traversed  by  the  current,  gets  hotter  and 
then  cooler,  hence  alternately  glowing  brighter  and  then  darker.  At 
the  moment  when  the  voltage  is  zero,  the  lamp  still  gives  out  a 
certain  amount  of  light,  since  sufficient  heat  is  stored  up  in  the 
filament  to  cause  light  to  be  emitted  during  the  short  period  that 
the  current  is  zero.  Nevertheless  the  flickering  light  resulting 
would  be  very  fatiguing  to  the  eyes.  When  the  alternations  follow 
one  another  very  quickly,  at  least  50  times  per  second,  the  fluctua- 
tions are  not  perceived,  and  a  current  of  this  periodicity  can  therefore 
be  used  for  electric  lighting. 

Our  second  experiment  consists  in  bringing  two  wires,  connected 
with  the  terminals  of  an  alternating  current  generator,  into  contact 
and  then  separating  them.  A  break  spark  will  be  produced,  like  that 
with  a  continuous  current,  and,  if  we  keep  the  two  ends  of  the  wire 
sufficiently  near  together,  we  may  get  a  continuous  arc.  Alternating 
currents  may  therefore  be  employed,  as  well  as  continuous  currents, 
for  feeding  arc  lamps.  The  same  systems  of  regulation  with 
which  we  became  acquainted  in  the  continuous  current  lamps — viz. 
series-,  shunt-,  and  differential-regulation — may  be  applied  to  alter- 
nating current  lamps  with  almost  equal  success.  There  are,  of 
course,  certain  differences  in  the  construction  of  the  lamps,  which 
we  shall  deal  with  later  on. 

The  property  of  the  continuous  current  arc  lamp,  that  the  positive 
carbon  is  sooner  consumed  than  the  negative,  is  naturally  not  found 
with  alternating  current  lamps,  since  the  carbons  are  alternately 
positive  and  negative,  hence  they  are  consumed  at  an  equal  rate. 
The  voltage  required  with  the  alternating  current  is  lower  (25-30 
volts)  than  that  necessary  for  the  continuous  current  lamps. 

The  flickering  of  the  light  when  the  number  of  alternations  is 
too  small  occurs  here  far  earlier  than  with  the  glow-lamp ;  whilst 
with  the  latter  we  get  a  fairly  constant  light  at  50  alternations  per 
second,  we  can  with  an  arc  lamp  hardly  use  a  current  of  less  than 
SO  alternations  without  getting  a  very  unsteady  burning  of  the 
lamp.  Thus  in  installations  where  arc  lamps  are  employed,  a 
current  of  not  less  than  80,  but  generally  100,  and  in  this  country 
frequently  200,  alternations  per  second  is  employed. 

With  a  continuous  current  a  magnetic  needle  was  deflected  by  a 
current  flowing  through  a  wire,  and  an  iron  rod  surrounded  by  a  coil 
was  magnetized  as  soon  as  a  current  flowed  through  the  latter.  In 
making  the  first  of  these  experiments  with  alternating  currents,  we 
are  unable  to  observe  any  deflection  of  the  magnetic  needle.  If  we 
watch  the  needle  very  attentively  we  find  that  it  vibrates.  On 
reducing  the  speed  of  the  machine  which  supplies  the  current,  so  as 
to  get  but  a  few  alternations  per  second,  say  two  or  three,  we  observe 
that  the  needle  swings  from  side  to  side.  As  often  as  the  current 
changes  its  direction,  just  as  often  the  needle  alters  the  direction  of 


ALTERNATING  CURRENTS  223 

its  deflection.  If,  however,  the  number  of  alternations  is  greater,  say 
twenty,  thirty,  or  more,  then  the  eye  cannot  any  longer  follow  the 
quicker  and  shorter  oscillations  of  the  needle,  and  only  the  small  and 
rapid  vibrations  of  the  needle  about  its  position  of  rest  can  be 
observed. 

If  we  wind  a  coil  of  wire  over  an  iron  core  and  send  through  it 
an  alternating  current,  the  core  will  be  magnetized  like  a  core 
surrounded  by  a  continuous  current.  It  will  also  become  able  to 
attract  pieces  of  iron  and  keep  them  fast.  In  carrying  out  these 
experiments  with  alternating  currents  we  observe  two  secondary 
phenomena,  which  we  do  not  observe  with  continuous  currents. 
Firstly  there  is  a  loud  humming  noise,  and  secondly  both  the 
magnetized  and  the  attracted  iron  become  strongly  heated.  The 
heating  we  shall  deal  with  later  on.  The  noise  may  readily  be  under- 
stood from  the  nature  of  an  alternating  current.  At  the  moment  the 
strength  of  the  current  passes  the  zero  line,  the  attractive  force 
ceases,  and  the  iron  pieces  tend,  and  even  begin,  to  fall  off  the  core. 
The  falling  very  quickly  ceases,  since  a  very  brief  time  afterwards 
the  current  increases  and  the  iron  is  again  attracted.  This  pro- 
ceeding, which  is  repeated  as  often  as  a  change  of  the  direction  of  the 
current  occurs,  causes  naturally  a  corresponding  noise  or  a  sound, 
the  pitch  being  higher  or  lower  according  to  the  number  of  the 
alternations. 

As  it  is  with  the  magnetic,  so  it  is  with  the  electro-dynamic 
effects.  If  through  a  fixed  and  a  movable  coil  we  send  the 
same  alternating  current,  we  observe  the  attraction  or  repulsion  as 
with  a  continuous  current  (see  p.  61,  Fig.  57).  This  is  easily 
understood.  Assuming  that,  at  any  instant,  the  current  in  the  two 
coils  have  the  same  direction,  then  these  coils  attract  each  other.  At 
the  same  instant  as  the  current  changes  its  direction  in  one  coil, 
it  will  also  do  so  in  the  other  coil.  Thus  the  two  coils  are  again 
traversed  by  currents  in  the  same  direction  and  attract  each  other. 

If,  on  the  other  hand,  we  send  through  one  of  the  two  coils  a 
continuous,  and  through  the  other  one  an  alternating  current,  then 
we  shall  observe  neither  an  attraction  nor  a  repulsion,  but  only 
a  little  vibration  of  the  coils,  since  the  first  impulse  of  the 
attraction  is  immediately  followed  by  the  opposite  impulse  of 
repulsion,  and  these  actions  continue. 

Next  let  us  try  to  get  a  chemical  effect  with  an  alternating  current. 
For  this  purpose  we  have  to  connect  the  two  electrodes  of  a  voltameter 
(see  Fig.  5)  with  the  slip-rings  of  an  alternator.  We  can  observe 
then  a  production  of  gas  at  both  poles,  although  not  at  one  pole  oxygen 
and  at  the  other  hydrogen,  as  is  the  case  with  continuous  current; 
but  at  both  electrodes  equal  quantities  of  the  explosive  gas,  consisting 
of  a  mixture  of  oxygen  and  hydrogen,  are  liberated.  With  an  alter- 
nating current  each  pole  is  first  positive,  and  immediately  afterwards 


224  ELECTRICAL  ENGINEERING 

negative,  so  that  a  bubble  of  oxygen,  evolved  from  one  pole,  will 
immediately  be  followed  by  a  bubble  of  hydrogen,  this  by  a  bub- 
ble of  oxygen,  and  so  on. 

Although  we  thus  get  chemical  effects  with  an  alternating  current, 
it  is  impossible  to  separate  the  elements  of  a  substance.  For  electro- 
lytic purposes  and  for  electro-plating,  where,  with  the  aid  of  the 
electric  current,  we  wish  to  separate  metals  from  metal  solutions — for 
instance,  silver  from  a  silver  solution,  or  copper  from  a  copper  solution 
— alternating  currents  cannot  be  employed.  It  is  also  obvious  that 
alternating  currents  cannot  be  employed  for  charging  accumulators. 

No  magnetic  or  chemical  effects  of  an  alternating  current  can 
be  observed  if  the  number  of  alternations  per  second  is  extremely 
great — say,  for  instance,  many  thousands.  Then  the  molecules  of 
iron  or  of  the  liquid  have  not  sufficient  time  to  follow  the  very 
rapidly  changing  pulsations,  which  tend  to  drive  them  at  one 
instant  in  one  direction,  and  at  the  next  instant  in  the  opposite 
direction. 


Current  Strength  and  Voltage  of  an  Alternating 

Current 


We  can  measure  the  strength  of  an  alternating  current  by  means 
of  its  various  effects;  for  instance,  its  heating  or  magnetic  effects.  It 
is,  however,  necessary,  before  dealing  with  the  different  methods  cf 
measurement,  to  make  clear  the  meaning  that  electrical  engineers 
attach  to  the  strength  of  an  alternating  current,  since  the  latter 
varies  between  its  maximum  positive  value,  zero,  and  its  maximum 
negative  value.  In  speaking  about  the  current  strength,  we  gene- 
rally do  not  mean  its  maximum  value.  By  an  alternating  current 
of  1  amp.  we  understand  a  current  which  would  cause  the  same 
heating  effect  as  a  continuous  current  of  1  amp.  This  adopted  value, 
also  called  the  effective  or  virtual  current,  is  naturally  a  mean  value 
only.  The  maximum  value  of  an  alternating  current  is  1.41  times 
as  great  as  the  mean  value;  or,  in  other  words,  the  effective  current 
is  equal  to  about  two-thirds,  or,  speaking  more  exactly,  to  0.707  of 
its  maximum  value. 

A  hot-wire  instrument  shows  the  right  current  both  for  alter- 
nating and  continuous  currents,  since  its  reading  depends  on  the 
heating  effect.  This  follows  from  the  definition  of  the  strength 
of  an  alternating  current;  for  the  deflection  of  1  amp.  on  the  hot- 
wire ammeter,  tells  us  that  the  measured  alternating  current 
produces  in  the  instrument  the  same  heating  effect  as  a  continuous 


ALTERNATING  CURRENTS 


225 


current    of    1    amp.,  with   which    the    instrument    has    been    cali- 
brated. 

Exactly  the  same  meaning  is  attached  to  alternating  voltage.  By 
effective  or  virtual  voltage  of  an  alternating  current  we  understand  the 
voltage  of  an  equivalent  continuous  current,  which  produces  the  same 
heating  effect,  in  a  given  ohmic  resistance,  as  the  alternating  current. 
A  glow-lamp  manufactured  for  110  volts  continuous  current  will 
therefore  glow  with  equal  light  if  switched  on  to  110  volts  alternating 
current,  although  the  instantaneous  values  of  the  alternating  pressure 
v^ry,  at  each  half-alternation,  from  zero  up  to  nearly  1J  times  the 
effective  voltage,  that  is,  up  to  about  155  volte. 


Induction  Effects  of  an  Alternating  Current 

All  experiments  hitherto  carried  out  with  alternating  currents 
have  been  similar  to  those  with  continuous  currents.  We  must  now 
deal  with  effects  produced  by  alternating  currents,  which  are  not 
possible  at  all  with  continuous  currents. 

Let  us  wind  over  an  iron  core,  consisting  of  a  bundle  of  fine 
wires  or  iron  disks,  a  coil,  so  that  the  iron  core  projects  beyond 

the  coil.  Next  let  us  lay  on  the  top  of 
the  coil  a  metal  ring.  As  soon  as  an 
alternating  current  passes  through  the 
coil,  the  ring  is  knocked  upwards  as  if 
by  an  invisible  hand  (see  Fig.  219). 
It  floats  freely  in  the  air,  as  if  it  had 
no  weight,  and  gets  extremely  hot.  If 
now  we  open  the  circuit,  the  ring  falls 
back  on  the  coil,  and  gradually  cocls 
down. 

From  the  heating  and  motion  of  the 
ring,  we  conclude  that  an  electric 
current  has  been  induced  in  it;  from 
the  direction  of  the  motion,  we  may 
deduce  the  direction  of  the  current. 
We  know,  from  the  experiments  with 

the  electro-dynamometer,  that  currents  in  the  same  direction 
attract  each  other,  and  when  in  the  opposite  direction,  repel  each 
other.  Hence,  we  learn  from  the  constant  repulsion  of  the  ring,  that 
a  current  is  induced  in  the  latter  which  is  always  opposite  to  that 
of  the  coil.  Representing  this  in  a  diagram,  in  Fig.  220,  the  full 
line  shows  the  curve  of  the  original  (inducing  or  primary]  current, 
and  the  dotted  line  the  direction  of  the  induced  current.  Fig.  220a 
indicates  the  same  thing  in  another  way.  If,  at  any  moment, 
the  alternating  current  in  the  coil  is  directed  upwards,  then,  at 


Metal  Ring 


Coil 


FIG.  219. — Repulsion  of  Metal 
Ring. 


22G 


ELECTRICAL  ENGINEERING 


the  same  moment,  the  current  induced  in  the  ring  is  directed  down- 
wards; if  the  current  in  the  coil  changes  its 
direction,  then  the  current  in  the  ring  does 
the  same. 

If  the  coil  were  traversed  by  a  continuous 
current,  one  end  of  the  iron  core  would  be  a 
north,  and  the  other  end  a  south  pole,  and 
the  lines  of  force  would  therefore  continuously 
flow  in  one  and  the  same  direction  through 
the  core.  Since,  however,  the  magnetizing 
coil  is  traversed  by  an  alternating  current, 
the  magnetic  field  alters  its  direction  re- 
peatedly. Thus,  through  the  interior  of  the 
metal  ring,  which  lies  on  the  coil,  lines  of 
force  flow  that  continually  change  their 
direction.  These  lines  of  force  produce  in 
the  ring,  which  represents  a  winding  closed 
on  itself  (see  p.  67),  an  E.M.F.,  and  hence 
a  current,  of  continually  changing  direction, 
the  number  of  alternations  of  which  is 
naturally  equal  to  the  number  of  alternations 
of  the  primary  current. 

Exactly  the  same  action  which  arises  in 
the  metal  ring  or  the  secondary  winding 
prises  also  in  the  primary  coil  itself,  even  if 
there  is  no  secondary  winding  at  all.  Any 
winding  of  the  primary  coil  encloses  a 
magnetic  field,  the  intensity  and  direction 
of  which  is  perpetually  altered.  Thus,  in 
each  winding  of  the  primary  coil,  there 
must,  as  in  the  secondary  metal  ring,  be 
produced  an  E.M.F.  which  is  opposite  to 
the  original  one;  that  is  to  say,  there 
exists  a  back  electro-motive  force,  like  that  of  the  many  examples 
with  continuous  currents  we  have  considered;  as  in  the  cases  of 
the  electro-motor,  the  storage  battery,  and  the  arc  lamp.  The 
back  E.M.F.,  produced  by  the  inducing  effect  of  alternating  currents 
on  their  own  circuit  (thus,  by  self-induction),  causes  the  current 
flowing  through  the  coil  to  become  far  smaller  than  would 
be  calculated  by  Ohm's  law.  Obviously  the  back  E.M.F.  can 
never  be  equal  to  the  primary  E.M.F.,  since  in  this  case  no 
current  would  flow  through  the  coil,  the  iron  core  would 
therefore  not  be  magnetized,  and  at  this  moment  the  production 
of  the  back  E.M.F.  would  also  cease.  The  back  E.M.F.  remains 


FIG.  220. 


0-2= 
Primary 


0 


0-1= 

Induced 


Current 


Current 


FIG.  220«. 


ALTERNATING  CURRENTS 


227 


FIG.  221. 


— as  in  the  electro-motor — always  a  little  less  than  the  primary 
E.M.F. 

The  heating  of  the  secondary  metal  ring  in 
our  experiments  teaches  us  why  we  must  employ 
for  this  experiment  a  bundle  of  wires  or  disks 
instead  of  a  solid  iron  core,  and  why  the  solid  core 
we  employed  for  demonstrating  the  electro-magnetic 
effects  of  an  alternating  current  got  extremely  hot. 
Any  portion  of  an  iron  core  we  may  imagine  as 
consisting  of  many  short-circuited  iron  rings  (see 
Fig.  221),  and  in  all  these  iron  rings  currents  are 
induced  as  in  the  secondary  metal  ring.  For  this 

reason  the  iron  cores  of  all  alternating-current  apparatus  have — 
like  the  armatures  of  continuous-current  machines 
— to  be  made  of  insulated  iron  wires,  cr  from  thin 
iron  disks,  which  are  insulated  from  each  other  by 
sheets  of  paper  or  by  a  layer  of  varnish  (Fig.  222). 
Since  with  alternating  currents  a  far  greater  number 
of  alternations  generally  are  employed  than  take 
place  within  the  armature  of  a  continuous-current, 
dynamo,  the  subdividing  of  the  iron  core  has  to 
be  carried  out  further  with  alternating-  than  with 
continuous-current  armatures.  Whilst  with  the 
latter,  disks  of  0.02  inch  thickness  are  employed, 
the  thickness  is  generally  reduced  to  0.012  inch, 
and  even  to  0.008  inch  with  alternating-current, 
apparatus. 

i       Again,  the  bobbins  for  alternating-current  electro- 
magnets   must    never    be    complete  metal  bobbins. 

Whenever  metal  bobbins  are  employed,  they  have  to  be  made  with 
a  slit  (see  Fig.  223),  so  that  the  bobbin  itself 
cannot  serve  as  a  short-circuited  secondary 
winding,  and  eddy,  currents,  and  therefore 
heating,  is  avoided.  To  entirely  prevent  the 
production  of  these  currents  the  bobbins  are 
in  many  cases  made  of  insulating  materials. 


Transformers 

FIG.  223. 

The  effects  produced  by  alternating  cur- 
rents dealt  with  in  the  last  chapter,  are  of  the  utmost  importance  in 
practice.  These  effects  enable  us  to  produce,  without  using  any 
moving  parts,  an  E.M.F. ,  in  a  secondary  coil  which  is  wound  over 
an  iron  core,  providing  that  there  is  also  a  coil  (the  primary) 
traversed  by  an  alternating  current,  wound  over  the  iron.  The  voltage 


FIG.  222. 


228 


ELECTRICAL  ENGINEERING 


Secondary 


FIG.    224. — Ring  Transformer. 


produced  in  the  secondary  coil  may  have  any  value,  it  may  be  larger 
or  smaller  than,  or  equal  to.  the  voltage  of  the  primary  coil. 

The  open  iron  core,  employed  in  the  experiment  of  Fig.  219,  is 
not  employed  in  this  case.  Obviously  we  want  to  get  a  strong  mag- 
netic field  with  the  smallest  possible  magnetizing  current,  and  must 
therefore  provide  for  the  lines  of  force  a  closed  path  through  iron. 
With  the  dynamo,  having  a  movable  part,  an  air  gap  in  the  magnetic 
circuit  cannot  be  avoided, 
whereas  with  the  trans- 
former we  may  have  an 
entirely  closed  iron  cir- 
cuit, as,  for  instance,  the 
iron  ring  shown  in  Fig. 
224.  The  ring  looks  like 
a  gramme  armature. 
Whilst,  however,  with  the 
latter  the  lines  of  force 
enter  the  ring  from  out- 
side, and  the  ring  forms 
only  a  part  of  the  mag- 
netic circuit,  with  the 
transformer  the  lines  of  force  are  produced  within  the  ring  itself, 
and  are  closed  in  the  ring,  without  leaving  it.  The  secondary 
coil  may  be  placed  at  any  point  of  the  ring.  If  the  ends  of  the 
secondary  coil  are  disconnected,  then  an  E.M.F.  is  induced,  whereas, 
if  the  ends  are  connected  through  an  outer  circuit — say,  for  instance, 
by  lamps — a  current  will  flow  both  through  the  coil  and  the 
circuit. 

We  have  now  to  consider,  how  great  the  voltage  produced  in  the 
secondary  coil  will  be.  Let  us  assume  the  number  of  windings  on 
the  primary  coil  to  be  100,  and  that  it  is  connected  with  an 
alternating  ^supply  of  100  volts.  Let  the  secondary  be  first  of  all 
opened.  Then,  as  we  know,  the  back  E.M.F.  produced  in  the 
primary  coil  will  be  nearly  as  much  as  100  volts — say,  perhaps, 
99  volts,  or  even  a  little  more.  For,  since  the  lines  of  force  are 
flowing  entirely  through  iron,  we  want  only  a  small  number  of 
ampere-turns  for  magnetizing  the  iron.  Hence  a  very  small  pressure 
difference  between  primary  and  back  E.M.F.  is  required  for  sending 
through  the  coil  the  magnetizing  current  for  overcoming  the  ohmic 
resistance.  Since  now  in  the  100  windings  of  the  primary  ceil 
a  back  E.M.F.  of  nearly  100  volts  is  produced,  the  back  E.M.F.  cf 
each  winding  will  be  nearly  1  volt.  Any  winding  of  the 
secondary  coil  has,  however,  the  same  title  to  voltage  as  a  winding 
of  the  primary,  since  both  are  traversed  by  the  same  magnetic 
flux.  The  voltage  produced  in  any  winding  of  the  secondary 
coil  will,  therefore,  be  equal  to  nearly  1  volt.  If,  for  example, 


ALTERNATING  CURRENTS  229 

the  secondary  coil  consist  of  10  windings,  then  its  voltage  would 
be  about  10,  with  100  windings  about  100  volts,  with  1000  windings 
about  1000  volts,  etc.  The  voltages  of  the  secondary  are  to  those 
of  the  primary  coil  exactly,  or  nearly  exactly,  as  the  number  of 
windings  on  the  two  coils. 

Now  let  us  connect  the  ends  of  the  secondary  coil  with  an  outer 
circuit,  so  that  the  E.M.F.  of  the  secondary  coil  may  produce  a 
secondary  current.  Then  the  iron  core  will  no  longer  be  traversed 
by  the  primary  current  only,  but  also  by  the  secondary  current. 
The  latter  is.  as  we  have  learned  from  the  experiment  with  the 
metal  ring,  in  an  opposite  direction  to  the  primary  current;  it  tends 
therefore  to  demagnetize  the  iron  core,  and  to  weaken  the  flux 
of  lines  of  force.  As  soon,  however,  as  there  occurs  the  slightest 
weakening  of  the  flux,  the  back  E.M.F.  of  the  primary  coil,  which 
was  before  nearly  equal  to  the  terminal  voltage,  will  naturally 
decrease.  Even  if  the  back  E.M.F.  decreases  by  1  volt  only,  this 
will,  at  the  small  ohmic  resistance  of  the  primary  coil,  cause  a 
considerable  strengthening  of  the  primary  current.  Thus  through 
the  primary  coil  as  much  more  current  will  flow  as  is  necessary 
to  counterbalance  the  demagnetizing  effect  of  the  secondary  coil. 
If,  for  instance,  we  had  10  secondary  windings,  and  the  current 
taken  from  them  were  50  amps.,  then  500  secondary  ampere- turns 
would  cause  demagnetization.  Instantly  500  primary  ampere-turns 
would  result,  and,  since  the  number  of  windings  of  the  primary  coil 
is  100,  its  current  would  be  equal  to  |£{J-  =5  amperes. 

Such  an  apparatus  is  called  a  transformer,  because  it  enables 
us  to  transform  a  current  of  high  voltage  and  small  amperage  into 
one  of  low  voltage  and  great  amperage,  or  vice  versa.  It  regulates 
its  primary  current  consumption  according  to  the  current  taken  from 
its  secondary  side,  and  is  therefore  quite  as  excellent  an  automatic 
regulating  apparatus  as  an  electric  motor. 


Shape  of  Transformers 


The  ring,  as  shown  in  Fig.  224,  is  theoretically  the  best  shape  for 
a  transformer  core.  This  shape  has,  however,  the  disadvantage 
that  the  winding  of  the  coils  has  to  be  done  by  hand,  which  is 
rather  troublesome  and  expensive  work.  Hence  shapes  are  gener- 
ally employed  which  enable  us  to  use  machine- wound  coils.  In 
Fig.  225  such  a  shape  is  shown.  The  transformer  consists  of  a 
horseshoe-shaped  main  part,  built  up  from  thin  iron  disks  with 


230 


ELECTRICAL  ENGINEERING 


paper  between  them.  The  primary  and  secondary  coils  are  pushed 
over  the  limbs  of  this  part.  After  fixing  the  coils  on  the  open  end 
of  the  horseshoe,  a  straight  piece,  also  consisting  of  iron  disks  and 
paper,  is  pressed  to  the  top  of  the  limbs  and  fixed  by  means  of  screws. 
We  have  now  a  closed  magnetic  circuit  as  before,  but  of  a  rectangular 
shape.  The  iron  core  of  Fig.  225  is  obviously  not  quite  as  good  as 
that  of  Fig.  224.  In  the  latter  case,  the  magnetic  circuit  is  entirely 
through  iron ;  but  in  the  core  of  Fig.  225  there  is  between  the  main 
horseshoe  part  and  the  straight  end  piece  a  joint,  which,  though  it 
may  be  very  small,  still  represents  an  air  gap.  This  transformer 
requires,  therefore,  a  little  more  magnetizing  current  than  a  ring 


FIG.  225.— Transformer  with  Horseshoe- 
shaped  Iron  Core. 


FIG.  226. — Transformer  with  Coils 
subdivided. 


transformer,  and  will  also  have  a  greater  magnetic  leakage.  If  with 
this  rectangular  shape  we  fix  a  primary  coil  on  one  limb  of  the  horse- 
shoe, and  the  secondary  on  the  other,  then  we  are  able  to  observe  a 
considerable  difference  in  the  voltage  of  any  primary  and  secondary 
winding.  The  reason  for  this  is  that  all  the  lines  of  force  produced 
in  one  limb  do  not  pass  the  other  limb,  but  a  considerable  part  of 
them  leaves  the  iron  core  at  the  edges  and  joints  and  flows  through 
the  air.  To  prevent  the  disadvantageous  effect  of  the  magnetic 
leakage,  the  primary  and  secondary  coils  are  generally  subdivided 
into  a  number  of  smaller  coils,  alternately  placed  over  the  iron  core, 
as  shown  in  Fig.  226.  In  this  case  we  have  four  primary  and  four 
secondary  coils,  which  are  fixed  two  at  a  time  on  each  transformer 
limb.  Sometimes  the  internal  diameter  of  one  coil  is  larger  than  the 


ALTERNATING  CURRENTS 


231 


outer  diameter  of  the  other  coil,  so  that  on  each  limb  the  secondary 
coil  may  be  pushed  over  the  primary,  or  vice  versa  (see  Fig.  227). 

Another  transformer  shape  is 
shown  in  Fig.  228.  Here  the  coils 
are  wound  over  the  middle  iron 
core,  which  is  then  completed  by 
two  U-shaped  yoke-pieces.  The 
flux  of  lines  of  force  is  spread  over 
the  two  yoke-pieces.  The  working 
of  this  transformer  is  obviously 
quite  the  same  as  that  of  the  trans- 
formers described  before.  Practi- 
cally it  has  the  advantage  that  the 
coils  are  protected  by  the  two  yoke- 
pieces  against  mechanical  injuries 
and  are  enclosed  as  within  a  shell. 
In  no  part  of  a  transformer 
which  is  exposed  to  the  changing 

FIG.  227. — Transformer  with  Coils         magnetic  field  must  solid  iron  parts 
wound  one  on  the  other.  be  employed,  because  these  would 

be  dangerously  heated.     Hence  the 

bobbins  are  generally  made  from  insulating  materials.     Solid  iron 
bolts  and  castings  must  never  be  used  in  connection  with  the  iron 


FIG.  228. — Shell  Transformer. 


core.  For  the  constructive  part,  however,  they  may  be  employed, 
but  care  has  to  be  taken  to  prevent  them  from  being  traversed  by  a 
considerable  number  of  stray  lines  of  force.  In  Fig.  229  the  general 
construction  of  a  transformer  (Ferranti  type)  is  shown. 


232  ELECTRICAL  ENGINEERING 

Applications  of  Transformers 

The  transformer  is  of  the  utmost  importance  in  the  practical 
applications  of  the  alternating  current.  The  facility  of  change  of 
pressure  it  affords  has  given  it  an  important  place  in  electrical 
engineering. 

We  know,  from  what  we  have  learnt  about  mains  (see  page  46), 


FIG.  229. — Westinghouse  Oil-insulated  Water-cooled  Transformer 
2250  K.W.,  22,000  Volts. 

the  advantages  high  tension  offers  for  the  transmission  of  energy,  but 
we  are  aware  on  the  other  hand  how  dangerous  a  high-tension  main 
can  become  in  inhabited  rooms.  It  would,  for  instance,  be  possible 
to  generate  with  continuous  currents  voltages  of  some  thousands,  thus 


ALTERNATING  CURRENTS  233 

enabling  an  economical  transmission  of  energy  over  distances  of 
several  miles.  Since,  however,  the  consuming  apparatus,  such  as  arc 
and  glow  lamps,  can  only  be  manufactured  for  comparatively  low 
voltages,  a  series  connection  of  many  lamps  would  be  required. 
Further,  the  dangers  of  high-tension  circuits  would  have  to  be  carried 
into  each  room  in  which  a  lamp  is  used,  involving  the  special  pre- 
cautions which  are  specified  in  connection  with  high-tension  mains. 

The  alternating-current  transformer  allows  the  transformation  of 
high  tension  to  any  required  low  tension  in  a  very  simple  and  reliable 
manner.  It  does  not  require  any  attendance,  is  self-regulating,  and, 
since  in  an  apparatus  in  which  the  parts  are  ail  stationary  the  insula- 
tion between  high-  and  low-tension  coils  can  be  made  in  a  very 
perfect  manner,  a  transformer  is  safer  than  any  rotating  machine  can 
possibly  be.  From  the  secondary  terminals  of  the  transformer 
only  low-tension  cables  lead,  with  which  the  house  mains  are 
connected:  thus  no  special  provisions  have  to  be  made  in  installing 
lamps,  etc. 

If  we  wish  to  obtain  a  similar  transformation  with  continuous 
current,  there  is  nothing  left  but  to  employ  a  high-tension  con- 
tinuous-current motor,  which  drives  a  generator  supplying  low- 
tension  current.  Continuous-current  converters  require,  it  must 
be  remembered,  since  they  are  rotating  machines,  attendance  and 
regulation.  Further,  their  efficiency  is  far  lower  than  that  of 
stationary  alternating-current  transformers  of  the  same  output. 

Sometimes  alternating-current  transformers  are  employed  for  the 
transformation  of  low  into  high  tension.  Nowadays  alternating- 
current  generators  for  2000-5000,  and  even  10,000  volts,  can  easily 
be  manufactured.  For  power  transmission  on  extremely  long  dis- 
tances, however,  voltages  up  to  30,000  and  even  more  are  employed. 
It  is  then  generally  preferred  to  produce  in  the  generators  currents 
of  comparatively  low  voltages ;  to  transform  these  currents  by  means 
of  transformers  into  the  high  voltage  required,  lead  this  high  tension 
to  the  places  of  consumption,  and  there  step  it  down  again  by 
transformers  to  a  pressure  low  enough  to  be  used  without  danger 
to  life. 


Phase-Difference 

Not  only  in  transformers,  but  also  in  all  alternating-current 
circuits,  self-induction  causes  specific  phenomena.  We  know  that 
any  wire  traversed  by  an  electric  current  produces  round  it  a 
magnetic  field,  the  lines  of  force  being  in  circles  (see  Fig.  17).  With 
continuous  currents  this  magnetic  field  is  stationary  and  uniform 
as  long  as  the  current  does  not  alter  its  strength.  The  field  of  a 
direct  current,  therefore,  does  not  exert  any  reaction  on  the  current 
itself,  since,  as  we  know,  we  must  have  an  alteration  of  the  field 


234 


ELECTRICAL  ENGINEERING 


intensity  to  produce  induction  effects.  On  the  other  hand,  the  field 
produced  by  an  alternating  current  changes  its  direction  and  strength 
continually,  thus  inducing,  both  in  the  conductor  itself  and  in  all 
neighbouring  conductors,  electro-motive  forces.  With  straight  con- 
ductors, in  the  neighbourhood  of  which  there  is  no  iron,  the  electro- 
magnetic whirl  of  force,  and  hence  the  E.M.F.  of  self-induction,  is 
comparatively  small.  If,  on  the  other  hand,  there  are  coils  in  the 
circuit,  especially  if  they  have  cores  of  iron,  the  influence  of  the  self- 
induction  on  the  circuit  is  considerable.  The  E.M.F.  of  self-induction 
may,  of  course,  be  also  represented  by  a  wave  line,  like  any  alternating 
current  voltage  and  alternating  current  strength,  but  it  does  not 
reach  its  maximum  value  at  the  same  time  that  the  current 
strength  reaches  its  highest  value,  and  its  zero  occurs  at  a  different 
time  to  that  of  the  current. 

In  Fig.  230  the  course  of  an  alternating  current  is  shown  by  the 


FIG.  230. — Alternating-current  Curve. 


wave  line.  The  magnetic  field  naturally  reaches  its  maximum  value, 
its  zero  point,  and  its  minimum  (negative)  value  simultaneously  with 
the  current  by  which  it  is  produced.  If,  for  instance,  the  current 
reaches  the  zero  line,  then  there  are  no  magnetizing  ampere-turns,  and 
no  magnetic  field  can  exist.  When  the  magneti/.ing  current  reaches 
its  maximum  value,  then  the  strength  of  field  also  reaches  its 
maximum  value.  When,  on  the  other  hand,  the  magnetizing  current 
is  in  the  opposite  direction,  then  the  direction  of  the  lines  of  force 
must  also  be  in  the  opposite  direction.  The  E.M.F.  of  self-induction 
can  only  be  produced  with  an  alteration  of  the  magnetic  field.  The 
more  rapid  the  alteration,  the  stronger  the  E.M.F.  of  self-induction 
will  be.  Whether  the  field  itself  is  strong  or  weak,  or  whether  it  is 
directed  in  one  or  the  other  sense,  does  not  make  any  difference  at  all; 
the  essential  circumstance  being  only  the  rate  of  growth  or  decrease 
of  the  field. 


ALTERNATING  CURRENTS  235 

The  above  figure  represents  the  growth  and  decrease  of  the 
magnetizing  current,  and  therefore  also  the  growth  and  decrease  of 
the  strength  of  field.  Considering  the  figure,  we  observe  distinctly 
that  at  a,  c,  and  e — that  is,  at  the  highest  and  lowest  positions — the 
field  for  a  moment  does  not  alter  its  strength  at  all.  Up  to  a  the 
current  has  grown,  but  at  a  the  growing  of  the  current  stops  for  a  brief 
interval.  From  there  it  falls  again,  first  slowly,  then  quicker  and 
quicker.  The  inclination  of  the  wave  line,  and  hence  the  decrease  of 
the  current,  is  greatest  at  b,  where  the  current  passes  the  zero  line. 
On  the  current  falling  still  further,  the  inclination  of  the  wave  line 
becomes  less  steep,  and  the  fall  is  slower,  until  the  lowest  point,  c,  is 
reached.  At  this  moment  a  point  of  rest  occurs  again  for  a  moment, 
then  the  field  grows,  first  slowly,  then  more  quickly  up  to  d. 
Thence  it  continues  to  grow  up  to  the  highest  point  e,  but  the 
rate  of  increase  is  again  a  slow  one.  From  this  it  is  clearly 
seen  that  the  rate  at  which  the  current,  or  the  field  which  it 
produces,  changes  differs  from  point  to  point.  When  the  current 
reaches  its  maximum  value  there  is  no  field  alteration  at  all,  and 
when  the  current  passes  the  zero  line,  the  field  changes  at  the 
most  rapid  rate. 

We  may  compare  this  with  the  differences  in  the  length  of  day 
and  night  at  the  different  seasons.  In  winter  and  summer,  when 
the  days  last  eight  hours  less  or  more  respectively  than  the  nights, 
the  alteration  in  length  from  one  day  to  another  is  hardly  perceptible; 
whereas  in  spring  and  autumn,  when  the  days  and  nights  are  almost 
equal,  the  alteration  in  the  length  of  consecutive  days  is  very  ap- 
parent. 

The  E.M.F.  of  self-induction  depends  on  the  rate  of  the  alteration 
of  the  field.  Hence  it  is  greatest  when  the  current  passes  the 
zero  line,  decreases  with  an  increasing  current,  and  becomes  nil  as 
the  current  reaches  its  maximum  value.  For  determining  the 
direction  of  the  induced  E.M.F.  we  have  only  to  consider  that  it  is 
always  opposite  to  the  alterations  of  the  field,  thus  being  positive 
when  the  field  decreases,  and  negative  when  the  opposite  is  the 
case.  This  rule  enables  us  to  draw  a  line  in  the  form  of  a  wave, 
representing  the  E.M.F.  of  self-induction  (see  Fig.  231).  A  glance 
at  this  diagram  shows  that  the  E.M.F.  of  self-induction  is  a  quarter 
of  a  wave  or  a  quarter  of  a  period  behind  the  producing  current.  This 
signifies  that  the  current  has  at  any  definite  moment  a  maximum 
value,  which  is  reached  by  the  E.M.F.  of  self-induction  a  quarter  of 
a  period  later. 

This  is  the  case  with  the  theoretical  transformer,  the  secondary 
circuit  of  which  is  open.  The  transformer  is  then  not  loaded,  therefore 
through  the  primary  coil  only  a  small  magnetizing  current  flows,  and 
this  lags  a  full  quarter-period  behind  the  impressed  voltage. 


236 


ELECTRICAL  ENGINEERING 


With  continuous  currents  we  calculated  the  watts  required  in 
any  circuit  simply  by  multiplying  voltage  and  current,  or — 

Volts  Xamps.  =  watts. 

It  is  quite  different  with  alternating  currents.  The  power  used 
at  any  instant  is  still,  of  course,  determined  by  the  product  volts 
X  amps,  at  this  particular  moment,  but  we  must  never  forget  to 
multiply  together  the  voltage  and  current  that  belong  to  each  other. 
In  other  words,  the  product  of  the  simultaneous  values  of  current 
and  voltage  must  be  taken. 

Now,  just  at  the  moment  when  the  voltage  has  its  maximum 
value  the  current  is  zero,  and  when  the  latter  has  its  maximum  value 
the  voltage  is  zero,  the  product  of  voltage  and  current,  the  watts, 
thus  being  at  these  times,  in  both  cases,  without  value. 

This  fact  can  be  made  clearer  by  an   example  from  daily  life. 


FIG.  231. 


Imagine  a  workman  who  is  sometimes  diligent  and  at  other  times 
lazy,  in  an  untidy  workshop,  where  the  tools  are  frequently  lost.  If 
he  cannot  find  his  tools  just  at  the  moment  when  he  is  most  inclined 
to  work,  or  again,  if  he  discovers  them  when  he  is  inclined  to  be  lazy, 
he  will  not  in  either  case  do  useful  work.  The  phase-difference 
between  the  possession  of  tools  and  inclination  to  work  brings  about 
a  working  result  of  zero  value,  although  there  is  sometimes  inclination 
to  work  and  sometimes  these  are  tools.  If  the  "phase-difference" 
is  not  quite  as  great — that  is  to  say,  if  the  man  finds  the  tools 
just  before  he  has  lost  his  inclination  to  work,  the  result  will  not, 
of  course,  be  nil,  but  it  will  surely  be  smaller  than  if  the  possession 
of  tools  and  full  inclination  to  work  had  been  simultaneous. 

Similiarly,  the  electrical  effect,  the  watt  output,  is  smaller  when 
a  displacement  of  voltage,  as  regards  the  current,  exists,  and  will  be 
smaller  the  nearer  the  phase-difference  approaches  to  a  quarter- 
period.  If  the  current  has  only  magnetizing  work  to  do,  as  is  the 


ALTERNATING   CURRENTS  237 

case  with  a  theoretically  unloaded  transformer,  then  there  is  no  watt 
output. 

The  magnetizing  current  which  is  displaced  by  a  quarter-period 
from  the  voltage  is  therefore  called  a  wattless  current.  In  the 
case  of  a  theoretical  unloaded  transformer,  we  have  only  wattless 
current. 

The  reverse  of  a  wattless  current  is  a  watt  current — that  is, 
a  current  which  has  no  phase-difference  from  the  voltage.  If  the 
voltage  reaches  simultaneously  with  the  current  its  highest,  its  zero, 
and  its  lowest  value,  then  we  get  the  maximum  of  work  that  can 
be  done  with  these  current  and  voltage  values.  The  output  may 
then  easily  be  calculated  by  multiplying  the  effective  voltage  by  the 
effective  current.  With  a  circuit  without  self-induction  this  is  really 
the  case.  If,  for  example,  we  measure  the  effective  voltage  as  100 
volts,  and  the  effective  current  as  40  amps.,  then  the  output  is. 
4000  watts,  exactly  as  with  a  corresponding  continuous  current. 

A  circuit  absolutely  without  self-induction  does  not  exist,  but, 
frequently  the  self-induction  is  very  smdl — for  instance,  with  glow 
lamps.  If  with  the  secondary  coil  of  a  transformer  we  connect  a 
number  of  glow  lamps,  then  through  the  secondary  circuit  nearly  a 
pure  watt  current  flows.  Thus  to  the  wattless  magnetizing  current, 
which  was  in  the  primary  coil  before,  a  watt  current  will  be  added. 
The  resulting  current  now  flowing  in  the  primary  coil  is,  of  course,, 
neither  absolutely  in  phase  with  the  voltage  nor  displaced  by  a. 
quarter-period.  Its  displacement  becomes  smaller  the  more  the  second- 
ary of  the  transformer  is  loaded.  With  a  fully  loaded  transformer 
the  small  wattless  magnetizing  current  is  practically  negligible  when 
compared  with  the  large  watt  current,  so  that  a  phase-difference  can 
hardly  be  observed.  Hence,  if  the  fully  loaded  transformer  takes. 
300  amps,  at  a  voltage  of  100,  this  will  correspond  practically  with  30 
kilowatts. 

Our  discussions  about  an  unloaded  transformer  have  hitherto 
referred  to  the  theoretical  case.  With  a  commercial  transformer  the- 
phase-difference  is  not  really  a  quarter-period.  We  have  learned 
that  only  a  wattless  current — that  is,  one  which  does  not  produce 
any  .effect,  like  the  mere  magnetizing  current  of  the  primary  coil  of 
an  unloaded  transformer — has  a  lag  equal  to  a  full  quarter-period 
behind  the  voltage.  As  a  matter  of  fact,  even  in  transformers  with 
an  open  secondary  circuit,  secondary  currents  are  produced,  since  the 
separate  iron  disks  form  closed  circuits,  and,  even  if  they  are  very 
thin  and  of  high  resistance,  eddy  currents  flow  through  them.  These 
currents  act  like  those  produced  in  the  secondary  windings  when 
their  circuit  is  closed.  Now,  whenever  a  current  flows  in  the 
secondary  circuit  a  watt  current  enters  the  primary  coil.  It  will 
therefore  be  quite  clear  that  through  the  primary  coil  of  an  unloaded 
transformer  a  certain  amount  of  watt  current  must  flow.  The 


238  ELECTRICAL  ENGINEERING 

transformer  will  always  consume  as  much  energy  as  is  transformed 
by  the  eddy  currents  in  its  core  into  heat.  The  phase-difference 
between  current  and  voltage  of  an  unloaded  transformer  is  therefore 
always  somewhat  less  than  a  quarter-period,  and  the  watts  taken 
are  always  greater  than  zero,  but  far  less  than  the  product  of  voltage 
and  current. 

The  self-induction  of  a  coil  with  an  iron  core  may  be  used  with 
advantage  in  installations  of  arc  lamps,  so  as  to  avoid  loss  of  energy. 
If  we  connect  a  single  alternating-current  lamp,  requiring  a  voltage 
of  about  30,  with  110-volt  mains,  we  have  to  absorb  about  80  volts 
in  a  series  resistance.  An  8-amp.  lamp  consumes  8  amps.  X  30 
volts  =  240  watts.  In  the  series  resistance,  as  much  as  8  amps. 


FIG.  232. — Choking  Coil  (The  General  Electric  Company). 

X  80  volts  =  640  watts  would  be  lost!  Thus  the  dynamo  had  to 
supply  880  watts  for  this  single  arc  lamp  only.  If.  on  the  other 
hand,  we  employ,  instead  of  the  series  resistance,  a  ''choking  coil" — 
that  is,  a  coil  wound  over  an  iron  core,  similarly  to  a  small 
transformer,  but  with  a  single  coil  only  (see  Fig.  232) — then  in  this 
coil  a  back  E.M.F.  is  produced,  which  causes  a  great  phase-difference 
between  current  and  voltage.  The  current  will,  of  course,  in  this 
-case  have  to  be  again  8  amps.,  also  the  voltage  of  lamp  and  choking 
coil  together  will  be  110  volts;  but  the  watts  taken  will  be  far  less 
than  880 — perhaps  not  much  more  than  the  240  watts  required  by 
the  arc  lamp  itself.  Naturally  this  arrangement  cannot  be  used  with 
continuous  currents. 

The  property  of  self-induction  and  phase-difference  between 
(current  and  voltage  is  inherent  in  all  alternating-current  circuits, 
•especially  in  coils  with  iron  cores.  Hence  electro-magnetic  measuring 
instruments  show  different  deflections  with  continuous  and  alternating 
•currents  of  equal  strength.  If  they  be  used  for  alternating-current 


ALTERNATING  CURRENTS 


239 


work  they  must  be  calibrated  with  an  alternating  current  of  the 
same  number  of  periods.  For  the  E.M.F.  of  self-induction  is  much 
less  with  a  current  of  50  than  with  one  of  100  periods.  The  instru- 
ment will  therefore  be  incorrect  for  any  other  periodicity  than  that 
for  which  it  has  been  calibrated. 


Vector  Diagrams 

Let  us  draw  the  vector  diagrams  of  the  cases  just  cited.  Take 
the  case  of  a  voltage  applied  to  a  choking  coil  in  series  with  an  arc 
lamp.  Let  us  assume  the  arc  lamp  takes  8  amperes  at  30  volts,  the 
current  and  voltage  being  in  phase.  Let  us  assume  that  the  choking 
coil  is  entirely  inductance,  having  no  resistance  or  iron  loss.  The 
diagram  would  appear  as  in  Fig.  233. 


FIG.  233.— Vector  Diagram. 

In  this  figure  the  line  o-a  represents  in  length  and  phase  the 
square  root  of  mean  square  value  of  current,  that  is  8  amperes,  as  read 
on  an  ammeter.  The  flux  produced  in  the  reactive  coil  is  in  phase 
with  the  current,  since  current  produces  flux. 

We  have  shown  in  Fig.  217  and  in  the  text  covering  it,  that  the 
E.M.F.  produced  by  a  flux  is  90  degrees  away  from  the  flux.  Thus, 
o-d,  90  degress  ahead  of  o-a,  represents  the  E.M.F.  produced  by  the 
flux  in  the  reactive  coil,  which  in  turn  is  produced  by  the  current 
flowing  through  the  reactive  coil.  The  line  o-b  represents  in  length 
and  direction  the  value  of  the  E.M.F.  at  the  arc  lamp.  This  is  in 
phase  with  the  current  o-a,  since  it  is  assumed  that  the  lamp  is  non- 
inductive.  Thus,  the  product  of  o-b  and  o-a  gives  the  energy  in 
watts  taken  by  the  lamp  itself.  The  voltage  required  to  overcome 
the  voltage  o-d,  lost  in  the  reactance,  and  the  voltage  o-b,  required 
by  the  lamp,  is  now  to  be  determined.  This  voltage  is  not  the  arith- 
metical sum  of  o-b  and  o-d,  because  they  are  out  of  phase  with  each 
other,  as  shown  in  Fig.  233,  and  voltages  or  currents  can  only  be  added 


240  ELECTRICAL  ENGINEERING 

directly  in  alternating  circuits  when  they  are  in  phase.  How,  then, 
should  they  be  added?  It  can  be  shown  that  to  add  quantities  out 
of  phase,  it  is  necessary  to  -find  the  diagonal  of  the  parallelogram  whose 
sides  compose  the  two  values  to  be  added.  Thus,  in  Fig.  233,  the  line 
o-c  represents  their  vector  sum.  Thus,  o-c  represents  in  phase  and 
amplitude  the  value  of  the  E.M.F.  necessary  to  put  8  amperes  through 
the  arc  lamp  and  reactance  in  series.  It  can  be  seen  that  this  value 
is  much  less  than  the  actual  sum  of  o-b  and  o-d. 

This  figure  also  shows  that  the  E.M.F.  o-c  is  out  of  phase  with  the 
current  o-a,  by  the  angle  c-o-a.  This  angle  is  called  the  lag  of  the 
current  o-a,  behind  the  E.M.F.  o-c.  Inductive  circuits  cause,  as 
shown,  a  lag  of  current  flowing  into  them  behind  the  E.M.F.  applied 
to  them.  Take  the  case  of  the  Fig.  219,  but  first  without  the 
ring  on  the  core.  Let  us  assume  that  there  is  no  loss  in  the  iron  of 
the  core  or  in  the  copper  used  around  the  core.  Fig.  234  shows  the 


b 

FIG.  234.— Vector  Diagram.  FIG.   235. — Vector  Diagram  of  Trans- 

Coil  without  Iron.  former. 

diagram  of  E.M.F.  current.  Here  o-a  represents  the  current  flowing 
into  the  coil  and  o-b,  90  degrees  away,  as  has  been  shown,  the  applied 
E.M.F.  Now  place  the  ring  upon  the  core,  and  it  is  noticed  that 
the  current  into  coil  promptly  increases.  The  current  in  the  ring 
must  be  supplied  from  somewhere;  thus,  each  ampere  in  it  must  appear 
in  equivalent  amperes  (allowance  being  made  for  extra  turns)  in  the 
coil  itself,  since  into  it  only  can  energy  enter,  the  wire  supplying 
energy  being  connected  only  to  the  coil. 

Consider  Fig.  235.  Let  o-a  equal  in  amplitude  and  phase  the 
flux.  Let  o-b  in  phase  with  the  flux  represent  the  current  which, 
when  flowing  into  the  coils  of  the  magnet,  produces  the  flux.  We 
assume  no  iron  loss,  so  that  the  magnetizing  current  proper  is  only 
considered.  This  flux  produces,  when  alternating  through  the  pri- 
mary, an  E.M.F.  equal  to  o-f,  and  through  the  secondary  an  E.M.F. 
equal  to  o-c;  for,  as  has  been  shown,  the  E.M.F.  from  flux  is  90  degrees 
away  from  it,  and  the  E.M.F.  o-c  in  the  secondary  or  ring  appears  in 


ALTERNATING  CURRENTS  241 

the  primary  or  coil  as  o-g  equal  and  opposite  o-c  (allowance  being 
made  for  the  difference  of  turns  between  the  ring  and  the  coil) .  Assum- 
ing the  ring  itself  to  be  non-inductive,  the  current  flowing  in  it  is  the 
result  of  the  E.M.F.  o-c  and  in  phase  with  it,  that  is,  o-d.  This  current 
has  its  equivalent  and  just  opposite  to  it  in  the  primary  or  coil, 
as  has  been  shown,  and  hence  appears  in  the  diagram  as  o-f.  Thus, 
the  ring  E.M.F.  appears  in  the  coil  as  o-g,  arid  the  ring  current  appears 
in  the  coil  as  o-f.  Therefore  the  two  currents  which  must  combine 
as  a  single  current,  since  only  one  current  can  flow  in  a  wire  at  one 
time,  are  o-b  and  o-f,  and  the  two  E.M.F. 's  which  must  combine  to 
give  the  applied  E.M.F.  are  o-g  and  o-i.  The  latter  is  the  E.M.F. 
of  self-induction  of  the  primary  coil.  This  is  at  right  angles,  as  has 
been  shown  in  the  case  of  self-induction  E.M.F.,  to  the  primary  or 
coil  current  o-e.  But  the  combination  of  two  vector  quantities  is 
the  resultant  of  the  parallelogram  with  the  two  forces  as  sides;  thus, 
the  vector  sum  of  o-b  and  o-f  is  o-e,  which  gives  the  phase  and  ampli- 
tude of  the  current  flowing  in  the  coil.  And  the  vector  sum  of  o-g 
and  o-i  is  o-h,  which  is,  therefore,  the  applied  E.M.F.  upon  the  coil. 
An  inspection  of  the  figures  shows  that  the  current  flowing  into  the 
coil  o-e  lags  in  phase  behind  the  applied  E.M.F.  upon  the  coil  o-h, 
by  the  angle  h-o-e,  which  now  is  less  than  the  lag  in  Fig.  1966,  where 
the  ring  was  not  on  the  core.  Thus,  the  energy  given  to  the  ring 
brought  the  current  and  E.M.F.  applied  to  the  coil  nearer  in  phase. 
As  a  matter  of  fact,  the  energy  now  represented  is  the  product  of 
the  current  o-e,  and  the  proportion  of  the  E.M.F.  upon  it  o-k,  for 
energy  means  the  product  of  current  and  E.M.F.  when  in  phase. 
Examining  the  triangle,  k-o-h,  shows  the  cosine  of  the  angle  k-o-h 

equals  -=-,  as  has  been  explained  at  the  first  of  the  chapter.     Thus, 

okXoe  =  ohXcos  kohXoe,  or  energy  equals  product  of  E.M.F.  and 
current  and  cosine  of  angle  of  lag.  This  value  cosine  of  angle  of  lag 
of  a  circuit  is  called  the  power  factor.  When  there  is  no  lag  the  power 
factor  is  unity,  for  the  cosine  of  0°,  as  you  know,  is  1.  With  a  lag 
of  90  degrees,  the  power  factor  is  0,  and  the  energy  is  0,  since  the 
cosine  of  90°  equals  0.  This  diagram,  which  has  just  been  explained, 
is  that  of  the  alternating  transformer,  the  ring  being  the  secondary 
circuit  and  the  coil  the  primary.  It  deserves  careful  study.* 

In  order  to  calculate  in  volts  the  value  of  self-induction  of  any 
circuit,  certain  constants  of  that  circuit  must  be  known.  In  Fig.  235 
the  line  o-i  is  drawn  to  show  in  volts  the  self-induction  of  the  coil. 
We  will  now  proceed  to  show  just  how  to  calculate  this  voltage  having 
the  coil.  In  any  circuit  there  is  a  coefficient  of  self-induction  denoted 
"by  electricians  by  the  letter  L.  It  is  equal  to  the  maximum  of  the 

*  For  complete  discussion  of  the  design  and  operation  of  a  transformer  treated 
itfith  no  calculus,  see  Chap.  ITI,  Raymond's  "  Alternating  Current  Engineering  " 


242 


ELECTRICAL   ENGINEERING 


flux    wave    times    turns   of  the   circuit   divided  by   ampere   times 

ir^rkAnann  max.  flux  X  turns  ,.    . 

100,000,000,  or  -         vxmn  nnnnnn  •    This  is  expressed  in  a  unit  which 
dmp.  x  iuu,uuu,uuu 

has  been  given  the  name  of  Henry.  When  multiplied  by  2?rN,  when 
TT  equals  3.14159,  and  N  equals  cycles  per  second  of  the  circuit,  ohms 
are  obtained;  thus,  having  L  of  a  circuit,  the  ohms  inductance  equals 
2?rNL  and  the  volts  inductance  (o-i  of  Fig.  235)  equals  2?rNLI,  when 
I  equals  the  current  flowing  in  the  circuit.  The  calculation  of  L, 
that  is  the  flux  times  turns,  is  the  same  as  the  calculation  of  flux  in 
any  circuit  and  must  be  done  as  shown  in  the  first  of  this  book,  where 
it  was  shown  that  flux  equals  1. 258  X  ampere  turns  per  unit  length  of 
circuit  Xp,  the  permeability  of  the  circuit.  Consider  another  prob- 
lem as  follows:  What  would  be  the  diagram  of  currents  and  E.M.F. 
of  a  circuit  consisting  of  an  inductance  in  series  with  a  resistance 
having  upon  it  an  E.M.F.  applied?  The  circuit  would  look  like 
Fig.  236. 

-^MiffiSto 


FIG.  236.— Circuit  with  Inductance  and  Resistance. 

Let  the  inductance  equal  LQ,  and  the  resistance  equal  RQ.  Let 
the  cycles  of  the  circuit  equal  N  cycles  per  second  and  the  current 
equal  IQ.  Then  the  E.M.F.  consumed  by  the  reactance  LO  equals 
27rNLoIo>  and  by  the  resistance  equals  IR0.  We  will  now  draw 
the  vector  diagram  of  these  voltages  and  current. 


FIG.  237.— Vector  Diagram. 

Draw  o-a  equalling  in  phase  and  amplitude  the  value  of  the 
current  flowing.  Then  the  E.M.F.  used  up  by  resistance  due  to  this 
current  is  in  phase  with  this  current  and  represented  by  o-b  (thus, 
o&Xoa  equals  energy  loss  due  to  resistance).  The  E.M.F.  of  self- 
inductance  is  at  right  angles  to  the  current  and  is  thus  represented 
by  o-d.  The  total  E.M.F.  necessary  to  drive  this  current  o-a  through 
the  resistance  and  inductance  in  series  is,  therefore,  the  vector  sum 


ALTERNATING  CURRENTS  243 

of  o-b  and  o-d,  or  o-e.  Any  circuit  can  thus  be  analyzed  and  shown 
diagrammatically  by  bearing  in  mind  the  laws  which  have  been  ex- 
pressed. 

To  prove  that  2;rNL  equals  ohms  : 


It  has  been  shown  that  ~T=  —  —  equals  "square  root  of 


mean  square'7  voltage  of  the  sine  curve  of  E.M.F.  produced  by  an 
alternator  of  one  turn  on  its  armature  and  of  two  poles.  The  back 
E.M.F.  of  a  coil  of  n  turns  having  threaded  through  it  an  alternating 
flux  of  maximum  value  of  $  and  turns  n  and  cycles  N  (cycles  N  equal 
revolutions  per  second  of  an  alternator  of  two  poles,  as  has  been 
shown)  is 

2?rNj>  (max.)Xn  ^ 
\/2x  100,000,000  ~ 

In  this  case  the  flux  alters  instead  of  remaining  constant  and  the  turns 
revolving  in  it.  Since  motion  is  relative,  the  same  formula  held  for 
the  E.M.F.  produced  by  the  flux  alternation,  as  cycles  equal  N, 
though  turns  equal  n. 

In  this  coil  the  coefficient  of  self-inductance,  as  has  been  shown, 
equals 

T  =  <t>  (max.)  n  _ 

amp.  (max.)  +  100,000,000* 


From  (1) 


_,       (j>  (max.)  n  amp.  (max.)      2;rN 
amp.  (max.)  X  100,000,000  XV<f 


Substituting  (2)  in  (3)  gives 

^     LXamp.  (max.) 

~  —• 


_.      amp.  (max.)  ,  ,       ,  ,  . 

But—        ^_      -  equals,  as  has  been  shown,  square  root  of  mean 

square  amperes,  as  read  on  an  ammeter.  Hence,  E  (square  root  of 
mean  square)  equals  2;rNLI,  where  I  equals  square  root  of  mean 
square  amperes;  since  from  Ohm's  law  volts  equal  current  times 
resistance,  it  follows  from  the  equation  E  =  2;rNLX  I  that  2;rNL  equals 
ohms,  which  was  to  be  proved. 

Referring  again  to  Fig.  237,  the  line  o-e  represents  the  opposite 
of  the  flow  of  current  by  resistance  and  inductance.  It  is  a  fact  that 
in  any  right  angle  triangle  the  long  side  equals  the  square  root  of 
the  sum  of  the  squares  of  the  other  two  sides;  thus,  o-e  equals  the 

square  root  of  ob2  +  be*  .    But  b-e  equals  o-d.    Thus,  o-e  =^/2xnL2  +  R2. 


244 


ELECTRICAL  ENGINEERING 


This  value  is  called  impedance  and  represents  the  opposition  in 
ohms  to  the  flow  of  an  alternating  current  in  a  circuit  containing  the 
resistance  R  and  the  inductance  27rNL. 

Wattmeter— Power- Factor 

For  determining  the  watt  consumption  of  an  alternating  circuit; 
it  is  not  sufficient  to  measure  the  effective  voltage  and  current.  For 
this  purpose  it  is  therefore  necessary  to  employ  an  instrument  which 
at  any  moment  is  influenced  by  the  simultaneous  values  of  current 


FIG.  238.— Wattmeter. 


FIG.  239. —Two-wire  Single-ph* 
Integrating  Wattmeter. 


and  voltage,  i.e.  an  instrument  which  directly  indicates  watts.  Such 
an  instrument,  the  construction  of  which  is  shown  in  Fig.  238,  is  called 
a  Wattmeter.  It  is  similar  to  the  electro-dynamometer  mentioned 
on  page  60,  with  the  difference  only  that  it  is  not,  like  the  electro- 
dynamometer,  wound  with  wires  of  equal,  but  with  wires  of  different 
diameter.  The  wattmeter  essentially  consists  of  a  fixed  coil,  of  few 
windings  made  of  thick  wire,  through  which  (as  with  an  ammeter) 
the  main  current  pa«ses,  and  of  a  movable  coil,  with  a  few  windings 
of  fine  wire,  which  (like  a  voltmeter)  is  in  series  with  a  resistance,  and 
is  directly  connected  on  the  full  voltage.  To  prevent  any  phase-dif- 


ALTERNATING    CURRENTS  245 

f  erence  between  the  shunt-coil  current  and  the  voltage  producing  it,  the 
movable  coil  and  the  series  resistance  must  have  small  self-induction. 
Hence  there  must    (1)  be  no  iron  in 
the  apparatus,,  and  (2)   the  coils  of 
the  resistance  in  series  with  the  coil 
must    be    "  doubly  wound/'      as    is 
shown    in    Fig.   240.     A   winding    of 
this     kind     prevents     self-induction, 
since    to    any    winding     tending    to 
produce  a  field  in  a  definite   direction 
there     is     opposed     a     neighbouring       FIG.  240. — Spiral  without  Self- 
winding tending  to  produce  a  mag-  induction, 
netic   field  in  an  opposite  direction, 

so  that  no  magnetic  field  results.  The  shunt  coil  within  the  watt- 
meter itself  cannot,  of  course,  be  wound  in  this  way,  since  it  then 
would  be  unable  to  exert  a  directive  force.  It  possesses,  therefore, 
a  certain,  although  small,  self-induction,  because  the  coil  consists  of 
very  few  windings.  The  self-inductionless  series  resistance  has,  in 
addition,  an  important  influence  in  preventing  lag,  which  depends 
not  only  on  self-induction,  but  also  on  the  ohmic  resistance  of  the 
circuit. 

To  keep  the  fixed  and  the  movable  coils  always  at  the  same 
position  at  right  angles  to  each  other,  so  that  their  repelling  action 
cannot  be  weakened,  the  movable  coil  has  always  to  be  turned  back 
to  its  original  position.  For  this  purpose  in  the  centre  of  the  dial 
there  is  a  milled  head,  with  a  spiral  spring  attached  to  it  and  to  the 
movable  coil.  The  stronger  the  repelling  force,  the  greater  is 
the  angle  we  have  to  twist  the  spring  through  by  using  the  milled 
head  in  order  to  turn  the  movable  coil  back  to  its  zero  position. 
The  head  has  a  pointer  attached  to  it,  so  that  we  can  read  on  the  dial 
how  much  we  have  turned  the  head  and  hence  how  great  is  the  torsion 
on  the  spring.  The  dial  being  usually  divided  into  360  degrees,  it 
is  necessary  to  calibrate  the  instrument.  This  may  be  done  with  a 
continuous  current,  by  sending,  for  instance,  a  current  of  10  amps, 
through  the  main  coil  and  connecting  the  shunt  coil  with  its  series 
resistance  to  a  source  of  100  volts.  If  now,  to  bring  the  shunt 
coil  back  to  its  zero  position  (to  help  in  doing  this  a  small  aluminium 
pointer  is  fixed  to  the  shunt  coil,  and  is  bent  up  to  reach  the  dial), 
we  had  to  turn  the  knob  through  30°,  we  then  know  that  30° 
correspond  to  1  kilowatt,  thus  1°  corresponds  to  33  J  watts. 

The  force  with  which  the  movable  coil  is  repelled  or  attracted  by 
the  fixed  coil  depends  with  alternating  current  at  any  moment 
on  the  instantaneous  values  of  current  and  voltage.  Since,  as  we 
know,  the  product  of  instantaneous  voltage  X  instantaneous  cur- 
rent really  is  equal  to  the  instantaneous  power  in  watts,  the 
wattmeter  will  at  any  moment  indicate  in  a  correct  manner  the 


246  ELECTRICAL  ENGINEERING 

output  of,  or  the  watts  taken  by,  an  alternating-current  circuit.  If 
current  and  voltage  are  exactly  in  phase,  as  is,  for  instance,  nearly 
the  case  with  a  glow-lamp  circuit,  the  reading  on  the  wattmeter  will 
be  exactly  equal  to  the  product  of  the  voltage  and  current  as  indicated 
by  suitable  instruments,  such  as  a  voltmeter  and  an  ammeter  of  the 
hot-wire  type.  If,  for  instance,  in  a  glow-lamp  circuit  we  read 
on  the  voltmeter  100  volts  and  on  the  ammeter  10  amps.,  then  the 
wattmeter  will  indicate  1000  watts.  If  we  had  in  circuit  a  phase- 
difference  of  a  quarter-period,  the  wattmeter  would  stop  at  zero. 
The  voltmeter  would,  for  instance,  show  100  volts,  the  ammeter 
10  amps.,  and  the  wattmeter  nothing. 

The  product  of  volts  X  amps,  is  called  the  apparent  watts,  that 
indicated  by  the  wattmeter  is  the  real  or  effective  watts.  From  the 
ratio  between  the  real  and  apparent  watts  we  are  able  to  cal- 
culate the  phase-difference.  The  ratio,  that  is  the  number  we  get 
by  dividing  the  real  by  the  apparent  watts,  is  called  the  power 
factor.  With  an  inductionless  load  the  power  factor  is  equal  to 
unity,  with  an  inductive  load  it  is  smaller  than  unity,  and  with  a 
phase-difference  of  a  quarter-period  it  is  zero.  Instead  of  the 
expression  "power  factor,"  for  mathematical  reasons  the  expression 
cos  (/>  is  generally  preferred  (where  <j>  is  the  angle  of  lag  and  cos  <j> 
indicates  the  cosine  of  this  angle). 

If  the  power  factor  is  known,  we  can  even  without  a  watt- 
meter determine  the  real  watts  used.  If,  for  instance,  cos  0  =  0.9, 
then  with  a  current  of  10  amps,  and  a  voltage  of  100,  the  real  watts 
will  be  100X10X0.9=900  watts.  If  with  an  unloaded  trans- 
former, consuming  100  volts  and  40  amps,  cos  <£  =  0.3,  then  its  real 
consumption  =100X40X0.3  =  1200  watts.  With  a  fully  loaded 
transformer,  consuming  100  volts  and  300  amps.,  the  power  factor 
(cos  (f>)  might  be  equal  to  0.99,  its  real  consumption  being  then 
100X300X0.99=29,700  watts. 

There  are  instruments  for  measuring  directly  the  power  factor, 
which  are,  however,  not  often  in  use.  They  are  called  phasemeters. 

Commercial  wattmeters  which  read  directly  upon  their  dials  the 
reading  of  watts,  just  as  ammeters  or  voltmeters,  are  sold  by  leading 
manufacturers. 


CHAPTER  IX 


ALTERNATORS 

THERE  are  many  kinds  of  alternating-current  generators  or  alter- 
nators. The  simplest  we  became  acquainted  with  in  the  form  of 
the  "magneto-electric  machine."  A  Gramme  ring  may  also  be 
employed  as  an  alternator  armature.  Its  construction  is  then  still 
simpler  than  that  of  a  continuous-current  armature.  The  commutr  tor 
can  be  omitted,  and  two  opposite  windings  have  to  be  connected  by 
wires  with  two  slip-rings.  This  is  shown  diagrammatically  in  Fig.  241, 


FIG.  241. — Ring  Armature  with  Slip-rings. 

in  which,  for  the  sake  of  distinctness,  the  two  slip-rings  are  indi- 
cated by  circles  of  different  sizes.  If  the  windings  a  and  6,  with 
which  are  connected  the  slip-rings,  are  situated  just  in  the  neutral 
zone,  then  the  conductors  of  the  left  half  are  in  series,  and  also  those 
of  the  right  half.  The  two  halves  are  in  parallel,  and  we  get  at  this 
moment  the  largest  voltage,  the  same  as  would  continuously  appear 

247 


248  ELECTRICAL  ENGINEERING 

if  the  armature  were  built  for  continuous  current.  If,  however,  the 
windings  a  and  b  leave  the  neutral  zone  (as  shown  in  the  diagram), 
then  one  part  of  the  windings  of  each  half  is  under  the  influence  of 
the  north,  the  other  part  under  the  influence  of  the  south  pole,  and 
the  voltage  of  each  half  becomes  therefore  smaller.  If  the  windings 
a  and  b  are  horizontal,  then  in  each  half  there  are  as  many  wires 
under  the  influence  of  the  north  as  under  the  influence  of  the  south 
pole,  and  the  momentary  voltage  becomes  zero,  whilst  at  the  next 
moment  the  voltage  is  reversed.  As  the  armature  continues  to 
revolve  these  changes  of  pressure  are  repeated,  so  that  a  regular 
alternating-current  pressure  is  produced  between  the  two  slip-rings. 

Naturally  in  a  four-  or  multi-polar  magnetic  frame,  ring  arma- 
tures can  also  be  employed  for  producing  alternating  currents, 
provided  that  the  series  or  parallel  connections  of  the  windings  and 
the  connection  with  the  slip-rings  are  made  in  a  corresponding  way. 

Multi-polar  machines  are  generally  employed,  since,  to  obtain  the 
usual  periodicity  of  100  per  second,  or  6000  per  minute  with  a  2-pole 
machine,  a  speed  of  3000  revolutions  per  minute  would  be  required, 
whereas  with  a  4-pole  machine  but  1500,  with  a  6-pole  machine 
1000,  and  with  an  8-pole  machine  750  revolutions  per  minute  are 
necessary. 

For  exciting  the  field  of  an  alternator,  continuous  current  is  essen- 
tial, and  is  supplied  either  by  an  outer  source  of  current  or  by  a  special 
sniLll  continuous-current  machine,  coupled  directly  to  the  alternator. 

In  cases  in  which  a  Gramme  armature  is  employed  as  an  alterna- 
tor armature,  besides  the  slip-rings  there  is  frequently  fixed  on  the 
armature  a  commutator,  enabling  the  machine  to  supply  continuous 
on  one,  and  alternating  current  on  the  other  side.  The  continuous 
current  may  then  be  used  for  exciting  the  magnetic  field.  Such  a 
double-current  machine  is  shown  in  Fig.  242. 

Ordinary  continuous-current  drum  armatures  may  also  be  used 
in  this  manner  and  provided  with  slip-rings.  The  latter  have 
then  to  be  connected  with  two  armature  wires,  which  are  distant 
by  the  width  of  one  pole-shoe. 

There  are  other  drum  windings,  which  are  quite  different  from 
those  of  continuous-current  armatures,  and  only  serve  for  producing 
alternating  currents.  The  simplest  example  of  an  alternating-current 
drum  armature  is  the  Siemens  H  armature  (see  Fig.  64).  This 
armature  is  provided  with  a  single  slot  per  pole,  and  the  windings 
are  wound  as  a  coil  through  the  two  slots,  which  are  opposite  to 
each  other.  With  this  armature  all  the  conductors  employed  in 
inducing  E.M.F.  have  at  any  moment  equal  positions  in  the  magnetic 
field.  All  the  wires  are  either  in  the  neutral  zone,  or  in  any 
position  between  the  poles.  Thus,  with  this  winding  in,  all  the 
wires  are  at  any  moment  either  induced  equal  voltages,  or  none 
at  all. 


ALTERNATORS 


249 


We  may  also  express  this  as  follows: — With  a  drum-winding, 
which  is  wound  .like  a  continuous-current  winding,  in  the  series 
connected  conductors,  E.M.F.'s  are  induced,  which  are  not  in  the 
same  phase,  whereas  with  the  two-slot  alternating-current  winding 


FIG.  242. — Rotary  Converter  (British  Schuckert  Co.). 

the  E.M.F.'s  of  all  the  conductors  are  at  any  time  equal  in  phase. 
Thus  100  conductors,  wound  as  a  continuous-current  armature,  will 
not  be  as  effective  as  100  wires  wound  within  the  slots  of  a  2-slot 
alternating-current  armature.  On  the  other  hand,  we  can  obviously 
place  more  conductors  on  the  whole  arma- 
ture circumference  than  in  two  slots. 

Instead  of  a  single  slot  per  pole  there 
might  as  well  be  two  or  mere  slots,  as 
shown  in  Fig.  243.  But  it  is  clear  that 
such  a  winding,  even  if  there  are  many 
slots,  is  very  different  to  a  continuous- 
current  winding.  With  the  alternating- 
current  winding  the  armature  is  wound 
so  that  all  the  coils,  if  traversed  by  a  con- 
tinuous current,  would  tend  to  magnetize 
the  armature  in  the  same  direction. 

The  winding  diagram  of  a  4-pole  machine,  having  a  single  slot 
per  pole,  is  shown  in  Fig.  244.  Both  coils  must  be  connected  in 
series  in  a  suitable  manner.  The  windings  shown  in  Figs.  64,  243, 


FIG.  243. — Armature  with 
Three  Slots  per  Pole. 


250 


ELECTRICAL   ENGINEERING 


FIG.  244 . — Four-pole  Armature 
with  Single  Slot  per  Pole. 


and    244    are    all    open    windings,    whereas    the    drum    and    ring 
armatures  have  closed  windings. 

Since  the  alternating-current  arma- 

I  I  ture  does  not  require  a  commutator, 

the  attention  that  it  requires  is  much 

U^^=^>J  less,  and  the  wear  and  tear  of  the  slip- 

rings  is  generally  less,  than  that  of  a 
commutator.  Notwithstanding  the 
brushes  of  an  alternating-current 
dynamo  also  require  attendance,  since 
they  are  worn  through  friction  and 
have  to  be  readjusted  from  time  to 

r<^— -^1  time.     Sparking  might  even  take  place 

at  the  brushes  if  the  contact  with  the 
slip-rings  is  not  sufficiently  good. 

With      high  -  tension       generators 
brushes  and  slip-rings  must  be  avoided 
whenever  possible.     Now,  with  alter- 
nating-current   generators   it  is   quite  easy  to  build  the   armature 
as  the  stationary,  and  the  magnetic  frame  as  the  rotating,  part.    From 

the  stationary  part  the 
alternating  current  may 
then  be  taken  without 
slip  -  rings  or  brushes, 
merely  by  means  of  fixed 
terminals  and  cables. 
A  scheme  of  this  type 
is  shown  in  Fig.  245. 
The  Gramme  armature 
is  arranged  on  the  out- 
side, and  in  the  interior 
of  it  the  magnet  sys- 
tem rotates.  The  arma- 
ture is  divided  into  four 
quarters,  and  opposite 
points  are  connected  with 
each  other,  exactly  as  is 
the  case  with  an  ordinary 
ring  armature.  There  is 
naturally  no  difference 
.Fie.  245.— Four-pole  Inner-pole  Generator  in  the  inducing  action 
with  Ring-armature.  whether  the  armature  or 

the  field  rotates. 

Very  often  a  drum  winding  is  used  instead  of  a  ring  winding — for 
tthe  reason  that  the  fixing  of  the  armature  and  its  building  up  within  the 
casing  is  simpler.  Fig.  246  shows  the  general  construction  of  an 


ALTERNATORS 


251 


--,- 


FIG.  246. — Eight-pole  Inner-pole  Alternator 

(Brothers  Korting}. 


8-pole  machine,  having  two  slots  per  pole.  All  the  eight  coils  of  the 
armature  are  connected 
in  series,  and  wound 
clock  and  counter- 
clockwise alternately. 
Since  now  the  coils  are 
first  under  the  influ- 
ence of  a  north  and 
then  of  a  south  pole, 
this  winding  will  give 
a  proper  series  connec- 
tion of  all  the  induced 
electro-motive  forces. 

Such  inner -pole 
machines  with  sta- 
tionary armatures  are 
far  more  reliable  than 
machines  with  rotating 
armatures.  The  arma- 
ture wires  are  gen- 
erally threaded  through  entirely  closed  mica  or  insulating  press- 
pahn  tubes,  which  are  embedded  in  the  slots.  The  closed  tubes 
have  a  very  high  insulating  power,  so 
that  even  with  high  voltages  there  is 
no  fear  of  their  breakdown  and  the 
leakage  of  electricity  from  the  winding 
to  the  iron  part.  The  single  wires  do  not 
require  very  good  insulation,  since  the 
pressure  difference  between  the  wires  is 
comparatively  small. 

The  rotating  magnetic  field  must  be 
excited  by  a  continuous  current,  it  is 
therefore  provided  with  tw.O  .  slip-rings, 
by  means  of  which  the  continuous  current 
is  supplied.  For  excitation  a  low-voltage 
current  of  about  65  to  110  volts  is 
generally  used.  Thus,  slip-rings  and 
their  brushes  do  not  present  any 
danger. 

Shapes   of  slots   generally   used  with 

alternating-current  machines  are  shown  in  Fig.  247.  The  slots 
are  here  generally  far  larger  than  those  of  continuous-current 
machines.  They  are  either  open  or,  more  frequently,  nearly  closed, 
and  of  rectangular,  circular,  or  oval  shape.  With  high-tension 
generators  entirely  closed  slots  and  insulating  tubes  are  generally 
used. 


FIG.  247. — Different  Shapes 
of  Slots. 


252 


ELECTRICAL   ENGINEERING 


Another  method  of  machine  construction  is  shown  in  cross-section 
in  Fig.  248.    The  magnet  wheel  consists  of  two  halves     On   the 


circumference  of  each  half  are  provided  tongue-shaped  extensions. 


ALTERNATORS 


253 


arranged  so  that  in  the  spaces  of  the  right  half  the  extensions 
of  the  left  half  project,  and  vice  versa.  These  tongue  -  shaped 
extensions  represent  the  poles.  The  single  field  coil  is  enclosed  by 
the  two  halves  of  the  magnet,  and  thus  rotates  with  them.  It  tends 
to  produce,  in  the  direction  of  the  axis  of  the  magnet  wheel,  on  one 
side  north,  and  on  the  other  side  south,  polarity.  Thus  the 
tongue-shaped  extensions  on  the  left  become  of  north  polarity, 
those  on  the  right  of  south  polarity.  Since  now  the  extensions 


FIG.   249. — Inductor  Type  of  Machine   (Maschinenfabrik  Oerlikon). 


belong  alternately  to  one  and  the  other  half,  we  have  here  a  row  of 
alternating  north  and  south  poles,  like  the  magnet  wheels  previously 
described.  Both  these  types  of  alternating-current  machines  belong 
therefore  to  the  " alternating  pole  type." 

With  both  types  the  exciting  current  has  to  be  led  to  the  rotating 
part  by  means  of  slip-rings. 

The  formula  for  the  E.M.F.  of  an  alternator  has  been  shown  to 

be  E.M.F.  (virtual)  =  ^  QQQQQQ'  wnere  N=cycles  per  second,  n= 
turns  embracing  flux  <£,  which  are  connected  in  series,  where  N= 


254 


ELECTRICAL  ENGINEERING 


revolutions  of  alternator  per  minute  multiplied  by  the  number  of 
pairs  of  poles  and  divided  by  60. 

A  usual  winding  of  a  single-phase  alternator  armature  is  shown 
in  Fig.  250. 

Each  coil  may  have  as  many  turns  as  desired  to  produce,  when 
all  are  in  series,  the  proper  number  of  n  to  give  the  desired  E.M.F. 
There  are  many  forms  of  windings.  Often,  between  the  coils,  as 
shown  in  Fig.  250,  another  complete  set  of  coils  is  inserted  using  the 
same  armature  slots,  the  extra  coils  either  being  placed  beside  or 
above  in  the  slot  of  the  other  windings.  These  extra  coils  have  to 


FIG.  250. — Armature  Winding. 

be  wound  left-handed,  if  the  first  are  right,  to  put  the  E.M.F.  induced 
in  them  in  series  with  the  other  E.M.F.  By  this  \neans  the  armature 
surface  is  more  filled  with  wires  and  thus,  for  certain  voltages,  the 
winding  is  more  desirable.  The  poles,  cycles,  speed,  amperes,  and 
volts  regulate  what  type  of  winding  should  be  used.  Those  shown 
are  common.  Such  a  winding  as  shown  in  Fig.  250  gives  a  single- 
phase  alternating  E.M.F.,  as  shown  in  Fig.  217,  page  218.  The  wind- 
ing of  Fig.  241,  page  247,  also  gives  the  same  wave  of  E.M.F.,  as 
do  the  various  alternator  windings.  Suppose  in  the  alternator  as 
shown  in  Fig.  241,  which  from  the  collector  rings  as  shown  a  wave 
of  alternating  E.M.F.  is  obtained,  taps  to  the  winding  are  made  at 
c  and  d,  at  points  at  right  angles  or  180  degrees  away  from  a  and  6. 
Suppose  these  taps  are  connected  to  two  extra  collector  rings,  what 
E.M.F.  would  be  obtained  from  these  extra  rings?  Obviously  an 


ALTERNATORS 


255 


alternating  E.M.F.  would  result  independent  from  the  E.M.F.  at  the 
rings,  as  shown  in  the  figures.  Obviously  this  E.M.F.  would  be  alike 
in  value  to  that  at  the  rings  as  shown  in  the  figures.  But  this  E.M.F. 
differs  in  one  important  point.  That  is,  it  is  out  of  phase  90  degrees 
with  the  E.M.F.  from  the  rings  as  shown.  That  is,  when  the  E.M.F. 
for  a—b  taps  is  a  maximum  (which  occurs  when  the  taps  a—b  are 
vertical  in  the  figure)  the  E.M.F.  from  c-d  taps  is  0  (which  occurs 
when  the  taps  c-d  are  horizontal).  Thus,  such  an  alternator  produces 
what  is  called  a  quarter-phase  E.M.F.  This  machine  is  then  of  a  class 
called  polyphase  alternators.  The  E.M.F.'s  are  as  shown  in  Fig.  217, 
one  phase  producing  the  E.M.F.  shown  by  the  full  line  and  the  other 
the  E.M.F.  shown  by  the  dotted  line,  differing  in  phase  from  the 
first  by  90  degrees.  Fig.  280,  page  287,  shows  also  the  quarter- 
phase  relation  of  E.M.F.'s  or  currents.  If  the  taps  on  armature 
shown  in  Fig.  241  were  at  points  120  degrees  apart  instead  of  at 


FIG.  252. — Diagram  Quarter-phase  Alternator. 

right  angles  to  each  other  a  three-phase  E.M.F.  would  be  produced, 
the  maximums  of  the  three  E.M.F.'s  being  120  degrees  apart  in 
phase.  This  is  shown  in  Fig.  288,  the  three  phases  being  represented 
by  a,  b,  and  c,  differing  at  their  positive  maximums  by  120°.  A 
polyphase  (in  this  case  a  quarter-phase)  is  shown  in  Fig.  310. 

Fig.  252  shows  diagrammatically  a  quarter-phase  alternator,  the 
two  windings  a  and  6  being  shown. 

The  E.M.F.,  E  and  E'  are  produced  equal  to  each  other  and 
reaching  their  maximum  values  90  degrees  apart.  Fig.  253  shows 
similarly  a  three-phase  alternator,  with  coils  A,  B,  and  C  set  120 
degrees  apart  in  phase,  that  is,  to  have  any  coil  give  the  same  E.M.F. 
as  the  next  the  armature  must  turn  120  degrees. 

Another  way  of  showing  this  same  thing  diagrammatically  is  as 
in  Fig.  254. 

As  before,  the  coils  A,  B,  C  are  120  degrees  apart,  just  as  in 
Fig.  253.  There  is,  however,  a  different  connection  in  the  two 
cases.  In  Fig.  253  each  coil  produces  the  full  E.M.F.  of  the  alter- 
nator, while  in  Fig.  254  the  E.M.F.'s  are  the  resultants  of  the  E.M.F. 
of  one  coil  with  the  next,  the  resultants  being  120  degrees  apart,  as  the 
coils  are.  The  windings  of  Fig.  253  are  said  to  be  connected  delta, 


256 


ELECTRICAL  ENGINEERING 


from  their  resemblance  to  the  Greek  letter  A,  and  in  Fig.  254  to  be 
connected  Y  or  star-connected.  The  resultant  of  the  E.M.F.'s  A 
and  B  (any  two  E.M.F. 's  give  the  same  result)  made  by  the  parallel- 
ogram of  forces,  as  has  been  shewn,  equals  V  3  times  the  E.M.F. 


FIG.  253.— Diagram  Three-phase  Alternator,  Delta. 

of  _pne  of  them.  Thus,  the  E.M.F.  of  a  single  coil  A,  of  Fig.  253,  is 
\/3  (equals  1.732)  times  the  E.M.F.  of  a  single  coil  of  Fig.  254  (assum- 
ing, of  course,  the  coils  to  be  of  equa_l^ turns).  Thus,  to  change  from 
A  voltage  to  Y  voltages  divide  by  \/3. 

In  Fig.  254  the  current  in  the  coils  is  the  same  as  the  current  in 


FIG.  254. — Diagram  Three-phase  Alternator,  Star. 

the  lines  e  e  e  running  to  the  external  circuit.  This  current  is  called 
the  Y  current.  The  current  in  the  coils  of  Fig.  253,  however,  com- 
bines before  going  into  the  lines  e  e  e.  Thus,  the  current  in  the  coils 
of  Fig.  253  is  called  the  A  current.  The  current  in  the  lines  is  of 
course  as  before  the  Y  current;  the  Y  current,  being  the  combination 
of  the  A  currents,  is  larger.  It  is  V  3  times  A  current.  Thus,  we 
have  in  the  three-phase  circuits  and  machines  Y  currents  and  voltages 
and  A  currents  and  voltages  differing  by  the  factor  V3.  A  three- 
phase  winding  is  shown  in  Fig.  256. 

The  regulation  of  an  alternator  is  influenced  as  in  a  direct-current 
generator  by  the  resistance  drop  of  the  armature  and  by  the  armature 


ALTERNATORS  257 

reactance.  The  resistance  drop  is  calculated  just  as  in  a  direct- 
current  machine.  The  armature  reactance,  however,  is  a  different 
matter.  In  a  direct-current  machine  the  current  is  a  constant,  and 
the  flux  produced  by  it  is  a  constant,  and  no  self-induction  exists. 
Also  a  direct-current  generator  has  a  commutator  upon  which  the 
brushes  are  shifted  forward  with  their  demagnetizing  iniluence.  With 
an  alternator,  however,  the  armature  current  is  variable  (a  sine- 
curve  current) ;  hence  this  current  must  produce  a  variable  flux 
and  therefore  self-induction.  Also,  the  current  flowing  from  an 
alternator  need  not  necessarily  be  in  phase  with  the  E.M.F.  created. 
Hence,  the  maximum  of  the  current  may  occur  after  the  maximum 
of  the  E.M.F. 

The  maximum  E.M.F.  is  produced  (see  Fig.  241)  when  the  taps 
b-a  are  vertical.  The  current  may  not,  if  lagging,  be  a  maximum 
till  later,  as  shown  at  b-a.  When  vertical,  the  demagnetizing  action 
of  the  armature  is  vertical  between  the  poles,  just  as  in  a  direct- 
current  machine  with  the  brushes  at  neutral  point.  Thus,  one  pole 
tip  is  strengthened  and  the  other  weakened — not  constant  in  value, 
however,  but  naturally  pulsating,  due  to  the  armature  current  pul- 
sat  ng.  When  the  current  lags,  however,  and  the  maximum  comes 
as  in  the  position  a-b,  shown  in  Fig.  241,  there  is  a  component  actually 
opposing  the  flux,  this  being  similar  in  effect  to  the  shifting  of  the 
brushes  on  a  direct-current  generator.  Thus,  we  have  to  lower  the 
voltage  of  an  alternator,  the  ohmic  drop,  the  self-induction,  and  the 
armature  reaction.  These  must  be  overcome  by  extra  field  current. 
If,  instead  of  lagging  current,  the  current  is  leading,  then  the  armature 
reaction  tends  to  help  the  voltage,  and  the  field  current  may  have  to 
be  lowered  as  the  load  comes  on.  Condensers  and  synchronous 
motors  with  strong  field  excitation  produce  leading  currents. 

Due  to  the  pulsating  nature  of  the  armature  reaction  of  single- 
phase  alternators,  the  pole-pieces  must  be  laminated  to  keep  down 
the  eddy  currents  which  would  be  produced  by  the  alternating  flux 
of  the  armature  currents  near  them. 

To  find  the  efficiency  of  an  alternator,  various  losses  must  be 
determined  and  added  to  the  output.  The  ratio  of  the  output  to 
the  sum  of  the  output  and  the  losses  gives  the  efficiency;  that  is, 
the  ratio  of  the  useful  output  to  the  total  energy  generated.  The 
losses  are,  first,  friction;  second,  core  loss;  third,  I2R  of  field;  fourth, 
I2R  of  armature.  The  core  loss  should  be  determined  exactly  as 
has  been  described  for  a  dynamo,  page  130.  The  normal  core  loss 
corresponding  to  full  load  should  be  taken  at  a  field  in  the  alternator 
to  give  the  voltage  of  E  +  IR,  when  E  equals  the  operating  voltage 
of  the  alternator  and  R  equals  the  resistance.  At  first  thought  it 
might  be  assumed  that  instead  of  R  equalling  the  above,  the  impe- 
dance, which  is'V  R2  +  27rNL2,  as  has  been  shown,  should  be  used. 
But  it  must  be  remembered  that  the  flux  produced  by  the  armature 


258 


ELECTRICAL  ENGINEERING 


ampere  turns,  called  armature  reaction,  and  the  flux  produced  by 
the  induction  of  the  armature  proper,  combine  with  the  main  flux, 
producing  actually,  therefore,  but  one  flux.  This  flux  need  produce 
but  E  +  IR  to  give  E  at  the  terminals.  The  armature  reaction  and 
inductance  can  be  regarded  as  a  tendency  for  pulling  down  the  voltage 
met  by  the  field  ampere  turns.  Having  measured  the  resistance  of 
armature  and  field  circuits,  then  PR  losses  are,  of  course,  known, 
from  which  the  efficiency  of  the  alternator  is  known,  being  equal 
to  output  in  watts  divided  by  output  in  watts  plus  core  loss,  plus 
I2R  field,  plus  Ii2Ri  armature. 

The  curve  of  voltage  with  load  variation  of  an  alternator  is  shown 
in  Fig.  257.     As  the  load  increases  the  voltage  drops. 


FIG.  256. 

If  the  current  of  the  field  is  increased  to  keep  the  voltage  con- 
stant, the  curve  of  field  current  plotted  against  load  appears  as  in 
Fig.  258.  As  may  be  noted,  the  current  in  the  field  must  be  increased 
as  the  load  comes  on.  The  two  factors  that  tend  to  lower  the  voltage 
as  the  load  comes  on  are  resistance  and  armature  inductance.  The 
resistance  can  easily  be  measured.  How,  now,  should  the  inductance 
be  measured?  The  inductance  consists,  first,  of  the  demagnetizing 
effect  of  the  ampere  turns  of  the  armature.  On  a  pure  inductive 
load  of  90  degrees  the  armature  ampere  turns  act  directly,  opposing 
the  field  ampere  turns  and  in  the  same  magnetic  circuit  as  act  the 
field  ampere  turns.  Thus  on  90  degrees  lag  the  ampere  turns  of  the 
field  spools  must  have  subtracted  from  them  the  ampere  turns  of 


ALTERNATORS 


259 


the  armature  current.  On  no  lag  the  ampere  turns  of  the  armature 
act  to  produce  a  flux  at  right  angles  to  the  main  flux  flow  of  the 
poles.  Thus,  as  the  lag  is  increased,  the  effect  of  the  demagnetizing, 


Load 
FIG.  257. 


ampere  turns  increases,  swinging  around  until  at  90  degrees  lag  the 
armature  ampere  turns  are  actually  opposing.  Second,  the  pure 
inductance  of  the  armature  windings  themselves  has  its  separate 
influence  to  influence  the  E.M.F.  The  flux  path  of  the  ampere  turns 
is  not  the  same  as  that  of  the  main  flux  produced  mainly  by  the 
field  spools,  but  it  is  a  leakage  circuit  around  the  wires  themselves. 
If  the  wires  of  the  armature  are  embedded  in  slots,  this  path  would 
be  down  one  tooth,  across  underneath  the  slot,  up  the  next  tooth,, 
across  the  gap,  across  the  pole  over  the  slot,  across  the  gap  again 
to  the  starting-point,  completing  the  circuit.  As  has  been  shown  in 
Fig.  237,  the  self-induction  effect  is  at  right  angles  to  the  current.  If 
the  current  from  the  alternator  lags  90  degrees  behind  the  alternator 
E.M.F. ,  and  if  the  E.M.F.  of  self-induction  lags  also  90  degrees  behind 
the  current,  under  such  conditions  the  self-induction  E.M.F.  would 
exactly  oppose  the  main  alternating  E.M.F.  in  its  effect.  Thus,, 
just  like  armature  reaction,  the  self-induction  effect  swings  around 
into  exact  opposition  with  increasing  lag  of  alternator  current.  Due 
to  this  similarity,  a  test  to  combine  both  effects  has  been  suggested 
by  Charles  P.  Steinmetz,  which  most  engineers  now  use  to  obtain 
regulation.  The  method  consists  in  short-circuiting  the  alternator 
upon  itself  and  increasing  its  field  current  until  full  current  is  flowing 
in  the  armature.  Note  the  ampere  turns  in  the  field.  Under  these 
conditions  these  ampere  turns  are  exactly  opposing  the  armature 
ampere  turns  as  well  as  overcoming  the  exactly  opposing  induction 
of  the  armature.  This  is  true  since  when  short-circuited  the  armature 
current  lags  practically  90  degrees  behind  the  small  E.M.F.  induced  to. 


260 


ELECTRICAL  ENGINEERING 


produce,  through  the  short  circuit,  full-load  current.  Thus,  we  have  a 
direct  measure  in  ampere  turns  of  these  values.  Mr.  Steinmetz  gives 
the  name  of  synchronous  reactance  to  this  value. 

Having  now  found  this  value  for  a  given  alternator  (this  holds 
true  for  a  single-phase  or  polyphase  alternator),  it  should  be  used 
as  any  value  of  reactance.  Then  consider  the  use  of  an  alternator 
on  a  non-inductive  load.  To  calculate  the  regulation,  let,  in  Fig.  259, 


load 


FIG.  258. 


o-o  equal  the  ampere  turns  to  produce  the  normal  E.M.F.  of  the 
alternator  E,  plus  the  IR  drop  when  running  at  normal  speed  no 
load.  Let  o-d  equal  the  ampere  turns  of  synchronous  reactance 
determined  as  shown.  Then  the  resultant  of  them  equals  o-c,  which 
equals  the  ampere  turns  necessary  to  produce  normal  voltage  E  at 
full  non-induction  load.  If,  when  this  load  be  thrown  off,  the  field 
ampere  turns  be  kept  at  o-c,  the  voltage  would  naturally  rise  above 
E,  since  o-c  is  greater  than  o-b.  The  amount,  then,  the  voltage 
rises  divided  by  E  gives  the  regulation  of  the  alternator.  This 
method,  therefore,  serves  not  only  to  determine  the  regulation,  but 
gives  an  opportunity  to  find  out  the  necessary  ampere  turns  of  field 
to  give  full  load.  Since  it  is  not  always  practical  to  actually  load 
alternators  when  making  tests  of  regulation,  etc.,  this  method  is  very 
convenient  and  it  is  at  the  same  time  very  accurate. 

If  the  current  flowing  from  the  alternator  be  lagging,  the  diagram 
of  Fig.  259  appears  as  in  Fig.  260,  when  the  current  o-a  is  shown 
lagging  by  the  angle  a  and  behind  the  E.M.F.  o-b.  In  such  case, 
plot  0-6  as  before  equal  to  the  ampere  turns  necessary  to  produce 
the  voltage  E+IR  at  no  load;  plot  o-d  as  before,  equal  to  synchronous 
reactance  ampere  turns,  but  in  this  case  plot  them  at  right  angles  to 
the  current  vector  o-a,  since  induction  is  always  90  degrees  away  from 
the  current.  Thus,  in  this  case  the  resultant  o-c  is  greater  than  in 
Fig.  260,  showing  that  under  lagging  load  the  ampere  turns  required 
in  an  alternator  are  greater  than  under  non-inductive  load. 


ALTERNATORS 


261 


This  same  method  is  used  to  obtain  regulation  of  transformers. 
It  is  not  practical  to  read  direct  the  regulation  of  a  transformer,  so 
instead  the  synchronous  reactance  is  obtained  similarly  to  a  generator. 
In  the  case  of  a  transformer  it  is  short-circuited  upon  itself,  and  the 


FIG.  259  —Vector  Diagram.     Regulation  on  A.  C.  Generator. 


voltage  necessary  to  put  field  current  through  the  windings  is  read. 
This  voltage  is  then  a  measure  of  the  inductance  of  both  primary  and 
secondary  added  together.  Knowing  this  and  the  resistance,  and 
remembering  that  inductance  in  vector  diagrams  must  always  be 
plotted  at  right  angles  to  the  current  and  that  resistance  drop  must 
be  plotted  in  phase  with  the  current,  the  diagram  under  load  can  be 
plotted  just  as  has  been  shown  in  Figs.  259  and  260. 


FIG.  260.— Vector  Diagram.     Regulation  on  A.  C.  Generator  Inductive  Load. 

Another  type  of  alternating-current  machines  is  represented  by 
the  inductor  type  shown  in  Figs.  249,  261,  and  262.  As  may 
be  seen  from  the  illustration  (Fig.  262),  the  magnet  wheel  has  no 
winding  at  all.  It  has  on  each  side  five  (or  with  larger  machines 
more)  pole  pieces.  By  a  stationary  coil  fixed  in  the  casing 


262 


ELECTRICAL  ENGINEERING 


FIG.    261. — Armature  and  Exciting  Bobbin  of  Inductor  Machine  (Maschinenfabrik  OerLikon}. 


ALTERNATORS  '263 

the  rotating  iron  part  is  magnetized,  one  side  with  its  pole  pieces 
becoming  north,  the  other  side  south,  magnetic.  The  stationary 
casing  contains,  besides  the  exciting  coil,  two  armatures,  which 


FIG.  262. — Magnet  System  of  an  Oerlikon  Inductor  Alternator. 


are  built  up  in  the  usual  way  with  iron  disks,  forming  rings 
surrounding  the  pole  pieces  of  the  right  and  left  sides  respectively. 
The  two  armatures  are  provided  with  windings  in  slots.  A  small 
continuous-current  dynamo,  generally  fixed  beyond  one  of  the  bear- 
ings, supplies  the  necessary  exciting  current. 

The  course  of  the  lines  of  force  in  this  machine  is  as  follows :  The 
lines  of  force, produced  by  the  stationary  exciting  coil,  leave  the  pole 
pieces  in  one,  say  the  left,  side,  enter  the  left  armature,  and  pass 
through  the  case  -which  is  generally  made  of  cast  steel,  sometimes 
of  cast  iron, — flow  through  the  right  armature,  and  from  there  back 
to  the  pole  pieces  of  the  right-hand  side  of  the  magnet  wheel.  Thus 
with  this  machine  the  wires  are  not  alternately  under  the  influence 
of  a  south  and  a  north  pole,  but  the  wires  of  one  half,  say,  for  instance, 
those  of  the  left,  are  always  acted  upon  by  north  poles,  those  cf  the 
other  half  always  by  south  poles.  Hence  if  with  this  machine  we 
connected  the  armature  wires  in  the  same  way  as  we  did  with  the 
alternating-pole  machines — viz.,  always  two  wires  which  are  distant 
by  the  width  of  one  pole — the  resulting  E.M.F.  would  be  nil;  the 
reason  being  that  the  two  wires  connected  with  each  other  would  be 
under  the  influence  of  a  pole  of  the  same  name,  and  thus  their  E.M.F.'s 
would  act  against  each  other. 

To  avoid  this  we  must  not  lead  the  winding  from  one  pole  to 
the  next  one,  but  must  complete  each  coil  by  passing  the  winding 


264  ELECTRICAL  ENGINEERING 

through  the  space  between  poles  of  the  same  name.  The  separate 
vCoils  then  may  be  connected  in  series  as  usual. 

We  want,  therefore,  with  such  a  machine  twice  as  many  slots  as 
there  are  poles,  and  only  half  the  wires  are  at  any  moment  effective 
in  producing  an  E.M.F.  From  this  it  will  be  clear  that  this 
machine  is  heavier,  and  thus  more  expensive,  than  one  of  the 
:rotating-field  type  of  equal  output.  It  has,  on  the  other  hand,  the 
;ad vantage  of  the  absence  of  any  rotating  windings,  and  thus  of  any 
•slip-rings.  Since,  however,  the  rotating  windings  and  the  slip-rings 
of  a  rotating-field  machine  do  not  give  any  trouble,  this  advantage  is 
not  a  very  important  one. 

The  right  and  the  left  half  of  a  continuous-pole  type  represent, 
in  a  manner,  two  separate  machines,  but  we  may  as  well  connect 
their  windings  in  series  and  so  get  the  double  voltage. 

The  most  up-to-date  type  of  alternating-current  generator  is  that 
of  the  alternating-pole  type  with  radial  poles,  which  revolve.  (See 
Frontispiece.) 


B        C1 
FIG.  263  — Synchronizing  Lamp  Connections. 


Switching    in     Parallel     of    Alternating-current 
Machines — Synchronizer 

To  run  two  alternating-current  generators  in  parallel,  several 
conditions  have  to  be  fulfilled.  The  second  machine  must — as  in. 
the  case  of  continuous-current  machines, — be  brought  to  the  same 

voltage  as  the  first  one;  it 
must  run  with  exactly  the 
same  speed;  and  it  must, 
at  the  moment  of  switching- 
in  parallel,  be  equal  in  phase 
with  the  first  machine.  The 
exact  correspondence  of 
speed  and  phase  is  called 
"  Synchronism." 

With  mechanical  speed- 
measuring  devices  —  ta- 
chometers and  speed-counters — it  is  impossible  to  determine  the  speed 
as  accurately  as  is  necessary  for  this  purpose.  There  is,  however,  a 
very  ingenious  and  simple  device 
which  indicates  electrically  small  dif- 
ferences in  the  speeds. 

In  Fig.  263  the  two  double  circles 
represent  two  single-phase  alternat- 
ors, which  can  be  connected  by  means 
of  a  single-pole  switch  AA'.  In  par- 
allel with  the  latter  there  is  connected 
a  glow  lamp  which  is  able  to  stand 
double  the  voltage  of  either  of  the 
alternators.  When  the  switch  is  open 
there  is  a  closed  circuit,  in  which  the 
two  machines  and  the  lamp  are  con- 
nected in  series.  If  the  two  machines 
were  continuous-current  machines, 
there  would  be  only  two  possibilities : 
either  they  work  in  series,  so  that 
their  voltages  are  added,  or  they  act 
in  opposition,  so  that  the  resulting 
voltage  is  zero.  If  both  machines  were  designed  for  110  volts,  thert 
in  the  first  case  the  lamp  receives  220  volts,  and  burns  with  its  normal 
intensity.  In  the  second  case  the  lamp  does  not  glow  at  all.  On  the 

265 


FIG.  264. — Westinghouse  Synchro- 
scope. 


266 


ELECTRICAL  ENGINEERING 


other  hand,  with  alternating-current  machines  there  are  between  these 
two  extremes  many  other  possible  cases.  According  to  the  phase- 
difference  between  the  two  machines,  tfjl  voltages  between  double 
and  no  voltage  may  be  given  to  the  lamp. 

If  now  we  want  to  switch  the  two  machines  in  parallel,  we 
have  to  watch  the  lamp.  Supposing  that  machine  II.  is  running 
a  very  little  slower  or  quicker  than  machine  I.,  then  the  lamp 
will  glow  for  one  moment,  and  be  dark  the  next.  At  the 
instant,  when  the  voltages  of  the  two  machines  are  equal  in 
phase,  the  lamp  will  remain  dark,  and  at  any  other  period,  in  which 
the  phases  are  displaced  by  half  a  period,  the  lamp  will  burn  with 
its  maximum  intensity.  If  two  60-pole  machines  differ  in  their 
speeds  by  four  revolutions  per  minute,  the  nickering  of  the  lamp 
will  appear  240  times  per  minute.  In  this  state  the  machines  must 
naturally  not  be  switched  in  parallel,  but  the  steam-engine  of  the 
second  generator  must  by  some  means — say,  for  instance,  by  adjusting 
the  governor,  be  brought  to  the  right  speed.  The  nearer  the  alternator 
approaches  the  right  speed,  the  slower  the  flickering  will  become; 
and  when  it  is  very  slow,  we  can  use  the  moment  the  lamp  is  dark 
again  to  switch  the  machines  in  parallel.  The  machines  are  then  in 
the  same  phase,  and  will  remain  so,  since  if-  one  machine  tends  to 
slow  up  it  will  be  driven  by  the  current  of  the  other  machine. 

Instead  of  a  lamp  a  voltmeter  may  be  employed.  As  long  as  the 
voltmeter  pointer  swings  quickly  backwards  and  forwards,  the 
machines  must  not1  be  switched  in  parallel,  but  if  the  vibrations 
become  very  slow,  the  moment  when  the  pointer  is  at  zero  may  be 
used  for  closing  the  switch. 

The  arrangement  of  Fig.  263  has  a  disadvantage:  the  machines 
have  to  be  switched  in  parallel  at  that  moment  when  the  lamp 
indicates  no  pressure.  This  moment  is  rather  difficult  to  determine, 
since  a  110-volt  bmp  becomes  dark  long  before  the  voltage  is 
nothing,  generally  at  Lbcut 

A 


B' 


15  to  20  volts.  Hence  it 
may  happen  with  this 
arrangement  that  the 
machines  are  switched  in 
parallel,  whilst  there  is 
still  a  considerable  differ- 
ence between  the  two  vol- 
tages, and  a  sudden  rush 
of  current  be  caused. 

To  obviate  this  an 
arrangement  is  often 
employed,  which  diagram- 
matically  is  shown  in  Fig. 
265.  The  machines,  to  be  switched  in  parallel,  are  first  separated 


FIG.  265. — Synchronizing  Lamps  cross-con- 
nected. 


ALTERNATORS 


267 


by  a  2-pole  switch.  Two  glow  lamps,  each  of  the  voltage  of  one 
of  the  generators  are  in  cross-connection  with  the  two  machines, 
thus  one  lamp  is  connected  with  A  and  B',  the  second  with  B 
and  A'.  The  current  flows  from  the  terminal  A  of  machine  L, 
through  the  upper  lamp  to  terminal  B'  (b)  of  the  other  machine, 
through  this  machine  to  terminal  a  (A'),  from  there  through  the 
lower  lamp  to  the  second  terminal  B  of  the  first  machine.  If  both 
machines  are  in  phase,  A  is  equivalent  in  voltage  to  A',  and  B  to  B'; 
thus  the  lamp  switched  on  A  and  B'  will  glow  with  the  same  voltage 
—that  is,  with  a  single  generator  voltage — as  if  it  were  switched 
on  A  and  B.  It  is  exactly  the  same  with  the  second  lamp.  If  the 
machines  happen  to  be  exactly  opposite  in  phase,  then  A  is  equiva- 
lent to  B',  and  B  to  A';  thus  the  lamps  will  remain  dark.  At  any 
other  phase-difference  the  lamps  will  glow,  but  not  as  brightly 
as  when  in  phase.  Hence  the  switching  in  parallel  has,  with 
this  arrangement,  to  be  done  at  the  moment  when  the  lamps 
are  brightest,  which  point  can  be  far  better  observed  than  when 
they  are  dark. 

The    connections    described    can    only    be    employed    with    low 

voltages.     For  medium  voltages,  say  300-5GO.  it  will  be  necessary 

to  use,  instead  of  single  lamps,  groups  of  3-5  series  connected  lamps. 

With    still   higher   voltages    this   is   inadmissible.     Hence,    with 

high-tension  generators,  the  lamps  are  not  put  in  the  high-tension 

circuit,    but   small    trans- 
A'  formers     are      employed, 

to  the  low-tension  side  of 
which  the  lamps  are  con- 
nected. In  Fig.  266  the 
diagram  of  connections 
is  shown.  If  A  is  equal 
in  phase  with  A',  then 
the  low-tension  termi- 
nals of  the  transformers, 
viz.  a  and  a',  are  equal  in 
phase.  Since  now  a  is  con- 
nected with  6',  and  a  and 
a'  are  in  series  with  the 
lamp,  the  voltages  of  the 
low-tension  coils  of  the 
transformers  are  added, 

and  the  lamp  will  glow  with  its  maximum  intensity.  The  trans- 
formers are  generally  designed  so  as  to  produce  a  low-tension  voltage 
of  55.  If,  then,  the  machines  are  equal  in  phase,  so  that  the  low- 
voltages  of  the  transformers  are  added,  a  110-volt  lamp  will  just 
burn  with  its  normal  intensity.  The  procedure  for  switching  in 
parallel  is  exactly  the  same  here  as  with  the  previous  arrangements. 


FIG.  266. — Arrangement  of  Synchronizing 
Lamps  for  High-tension  Circuits. 


268 


ELECTRICAL  ENGINEERING 


The  action  of  two  alternators  in  parallel  can  be  shown  by  Figs. 

In  Fig.  267  the  lines  1-2  and  1-3  represented  the  E.M.F  's  of 
the  two  alternators  in  parallel.  They  are  drawn  beside  each  other 
but  in  reality  are  exactly  superimposed.  The  condition  represented 
by  Fig.  267  is  when  the  two  alternators  have  the  same  wave  shape, 
the  same  voltage,  and  the  prime  movers  (engines  or  water-wheels) 
run  at  a  constant  speed  throughout  each  revolution.  Under  these  con- 
ditions no  cross-current  flows  between  the  alternators,  but  each  does 
its  share  of  the  work.  Suppose  the  wave  shapes  are  different.  Then, 
as  the  wave  of  one  during  its  generation  becomes  bigger  or  smaller 
than  the  other,  a  current  will  flow  from  one  alternator  across  to  the 
other,  since  they  are  connected  directly  together,  the  path  of  the 


FIG.  267. 


current  being  thus  through  the  armature  of  one  machine  across  the 
connecting  wires  between  the  two  machines  and  then  through  the 
armature  of  the  second.  This  effect,  while  it  may  exist,  is  usually 
negligible,  and  so  will  not  be  discussed  here.  The  other,  that  is 
variation  in  speed  during  a  revolution,  is  more  serious  and  frequent, 
especially  with  engine  direct-connected  units.  Under  some  circum- 
stances the  engine  and  generator  may  swing  apart  during  a  single 
revolution.  This  effect  is  shown  in  Fig.  268.  The  two  voltages  1-2 
and  1-3  are  now  swung  apart  as  described  by  the  angle  2-1-3.  This, 
then,  now  equals  the  resultant  voltage  2-3  (completing  the  triangle)! 
which  is  free  to  create  current  through  the  windings  of  the  two  alter- 
nators, circulating  around  through  the  cross-connecting  or  buss  wires. 
The  line  1-3 'equals  2-3,  drawing  it  as  usual  in  either  diagram  to  a 
common  centre  (in  this  case,  point  1).  This  represents  the  free 
voltage.  The  current  from  this  voltage  equals  the  vector  1-3  divided 
by  the  sum  of  the  impedance  of  the  alternator  armatures  in  series. 
This  circuit  is  inductive,  since  the  induction  is  much  more  than  the 
resistance.  Thus,  the  current  flowing  lags  much  behind  the  E.M.F. 
and  the  current  for  1-3  equals  1-4,  lagging  behind  it  by  the  angle 
4-1-3.  But  this  brings,  as  can  be  seen,  the  current  1-4  apparently 
in  phase  with  the  E.M.F. 's  1-2  and  1-3,  and  thus,  since  E.M.F. 's 
and  currents  in  phase  represent  energy,  this  exchange  of  current 


ALTERNATORS  269 

represents  energy,  and  thus  there  is  a  prompt  tendency  by  the  current 
to  pull  the  alternators  together  again.  This  is  called  synchronizing 
action  and  is  what  keeps  alternators  in  multiple  from  falling  out  of 
step. 

Suppose  no  swing  action  exists  as  just  described,  but  one  voltage 
is  greater  than  the  other.  This  may  be  shown  by  Fig.  267  again, 
where  the  vector  1-3'  represents  the  difference  between  the  two 
E.M.F.'s  in  phase  with  them  in  this  case,  since  exact  synchronism  is 
assumed.  Again,  the  current  from  these  E.M.F.'s,  as  in  Fig.  268, 
lags  about  90  degrees  from  it  and  can  be  shown  by  the  vector  1-4. 
This,  however,  is  90  degrees  away  from  the  voltage  vectors  1-2  and  1-3, 
and  thus  does  not  represent  energy,  since  E.M.F.'s  and  currents  in 
phase  represent  energy,  and  90  degrees  apart  represent  no  energy. 
Thus,  the  current  does  not  tend  to  pull  the  alternators  together,  rep- 
resenting no  energy.  Hence,  if  alternators  in  parallel  do  not  take 
their  respective  portions  of  load,  altering  field  will  not  usually  help 
matters,  but  the  throttle  and  water  (in  case  of  water  pans)  must  be 
adjusted.  Also  in  removing  an  alternator  from  the  busses  by  pulling 
the  main  switch,  the  current  flowing  cannot  be  cut  down  by  lowering 
the  alternating  field,  since  this  may  actually  increase  the  current 
flowing  (being  cross-current,  not  energy  current,  however).  The 
arc  also  from  breaking  such  a  lagging  or  leading  current  is  much 
worse  than  with  an  equal  energy  current,  since  with  energy  current 
the  E.M.F.  and  current  pass  through  0  together,  whereas  with  lagging 
or  leading  current,  if  one  is  0  the  other  has  value  and  hence  gives  more 
sparks  at  whatever  part  of  the  wave  the  break  of  current  may  occur. 
(With  E.M.F.  and  current  in  phase,  the  arc  is  0  if  the  current  happens 
to  be  broken  as  the  wave  passes  through  0.) 

The  way  to  withdraw  one  alternator  from  a  group  is  to  lower  the 
driving  power  until  the  current  commences  to  lower,  keeping  the 
alternators  in  phase  (this  takes  care  of  itself)  and  the  E.M.F.'s  the 
same  as  the  other  alternators.  When  due  to  lowering  the  driving 
power  by  the  throttle,  the  current  dies  down  just  as  it  reaches  a  very 
small  value,  preferably  0,  the  switch  can  be  pulled  and  the  alternator 
taken  out  of  circuit.  With  high-tension  machines,  such  as  10,000 
volts,  this  method  is  desirable. 


CHAPTER  X 
ALTERNATING-CURRENT  MOTORS 

Synchronous  Motors 

ALTERNATING  currents  have  the  great  advantage  over  continuous 
currents  that,  in  the  stationary  windings  of  a  generator,  high 
voltages  may  be  produced  easily  and  without  danger,  and  this  high 
pressure  may  be  subsequently  lt stepped  down"  by  stationary 
transformers  to  a  conveniently  low  pressure. 

There  are  different  kinds  of  alternating-current  motors.  Our 
first  thought  will  naturally  be,  whether  we  cannot  use  an  alternating- 
current  generator  as  a  motor,  as  we  are 
accustomed  to  do  with  continuous-current 
machines.  Let  us  consider  this  case  by 
the  aid  of  Fig.  269,  which  represents  the 
simplest  type  of  an  alternator,  viz.  the 
Siemens  armature  with  a  single  armature 
winding  rotating  in  a  2-pole  field  excited 
by  continuous  current.  If  through  this 
winding  we  send  by  means  of  two  slip- 
rings,  a  current  in  the  direction  marked 
by  a  dot  and  cross  respectively,  then  the 
armature  will  tend  to  rotate  clockwise.  IG< 

Now  the  motor  wants   a    definite  time 

for  starting.  But  before  it  has  started  to  move,  the  current  has 
already  altered  its  direction;  thus  the  armature  now  tends  to  rotate 
in  the  opposite  direction.  With  a  current  of  100  alternations  per 
second  no  rotation  of  the  armature  will  take  place,  but  merely  a 
vibration  will  be  noticed,  just  as  we  have  seen  with  a  magnetic  needle 
surrounded  by  an  alternating  current.  This  motor  cannot,  therefore, 
be  made  to  start  by  an  alternating  current. 

Assume  now  that  we  are  able  to  keep  the  current  in  the  direction, 
as  marked  in  Fig.  269  until  the  armature  has  started  to  rotate  and 

270 


ALTERNATING-CURRENT    MOTORS  271 

has  made  half  a  revolution.  Whilst  the  wires  are  in  the  neutral 
zone  again,  let  us  reverse  the  current.  The  armature  now  possesses 
a  certain  amount  of  live  energy,  so  that  it  can  pass  the  dead 
points  which  occur  when  the  wires  are  in  the  neutral  zone.  After  the 
reversal  of  the  current  the  wire  which  was  previously  under  the 
influence  of  the  north  pole  will  now  be  under  the  influence  of  the 
south  pole,  and  vice  versd.  Since,  however,  the  current  has  altered  its 
direction,  the  rotation  of  the  armature  in  the  same  direction  will 
continue,  and  the  armature  will  therefore  rotate  more  rapidly. 
Obviously  we  must,  just  at  the  moment  the  wires  pass  the  neutral 
zone,  alter  the  direction  of  the  current,  or  the  rotation  cannot  be 
maintained. 

To  start  a  motor  in  this  manner  is  naturally  impossible,  since 
an  alternating  current  supplied  for  driving  a  motor  has  its  normal 
periodicity  from  the  beginning.  Nevertheless  we  have  learned  from 
this  consideration  that,  if  such  a  motor  be  once  brought  to  its  full 
speed,  it  can  be  kept  in  rotation  and  do  work.  Thus  we  must 
start  the  motor  by  some  auxiliary  power  before  switching  it  on  the 
mains,  and  bring  it  to  its  full  speed — that  is  to  say,  to  that  speed 
which  corresponds  to  the  number  of  alternations  of  the  current 
supplied.  If,  for  instance,  the  latter  makes  6000  alternations  per 
minute,  then  we  have  to  bring  the  armature  to  a  speed  of  3000 
revolutions  per  minute,  and  after  having  made  sure  that  the  neutral 
armature  position  coincides  exactly  with  the  change  of  direction 
of  the  alternating  current,  i.e.  that  motor  and  generator  are  "syn- 
chronous," we  can  switch  the  motor  on  the  source  of  current.  To 
ascertain  whether  motor  and  generator  are  in  synchronism  we  use 
a  synchronizer  as  described  at  the  end  of  the  last  chapter. 

"This  type  of  motor  is  called  a  synchronous  motor.  Any  alternate 
ing-current  generator  can  run  as  a  synchronous  motor.  The  speed  of 
a  synchronous  motor  is  quite  a  definite  one,  and  may  easily  be  found 
from  the  number  of  alternations  of  the  current  and  the  number  of 
poles  of  the  motor.  A  2-pole  machine  will  with  a  current  of  6000 
alternations  per  minute  run  with  3000  revolutions  per  minute,  and 
an  8-pole  motor  with  750  revolutions.  If  from  any  reason — say,  for 
instance,  a  heavy  overload  of  the  motor — its  speed  falls  off  but  as 
much  as  half  the  width  of  a  pole,  then  the  motor  is  almost  instantly 
stopped.  For,  while  the  armature  conductors  are  still  under  the 
influence  of  one  pole,  there  are  produced  forces,  due  to  the  change 
of  the  current  which  tend  to  drive  the  motor  in  an  opposite 
direction.  Thus  the  motor  is  subjected  to  a  powerful  braking 
action,  and  stopped  in  a  short  time,  while  consuming  a  large 
current. 

This  type  of  motor  has,  therefore,  two  considerable  disadvantages. 
It  requires  an  auxiliary  power  for  starting,  and  is  stopped  if, 
for  any  reason,  the  synchronism  is  destroyed.  It  may  be  compared 


272  ELECTRICAL  ENGINEERING 

to  a  novice  in  cycling.  He  cannot  by  himself  get  on  a  bicycle  and 
set  it  into  motion,  but  once  the  machine  is  brought  up  to  sufficient 
speed,  he  is  able  to  keep  it  from  falling.  If,  however,  he  is  impeded 
by  any  obstacle  in  his  run,  he  falls,  and  a  new  start  has  to  be  made 
with  the  help  of  an  assistant. 

Hence,  for  many  purposes,  synchronous  motors  cannot  be  em- 
ployed at  all — as,  for  example,  for  the  purpose  of  driving  shafts  in 
small  workshops  having  no  other  power  at  liberty  for  starting  the 
motor.  Likewise  a  synchronous  motor  cannot  be  employed  in  cases 
where  frequent  starting,  or  a  strong  effort  at  starting,  is  necessary, 
as  is  the  case  with  cranes,  lifts,  and  railways. 

On  the  other  hand,  the  synchronous  motor  has  certain  advantages. 
First  of  all,  the  speed  of  the  motor  is  very  uniform,  a  property  very 
desirable  in  many  cases.  Further,  the  synchronous  motor  has  a 
decided  advantage  over  all  other  alternating-current  apparatus,  in  the 
fact  that  no  phase-difference  between  voltage  and  current  is  caused 
by  it.  We  shall  later  on  deal  with  other  alternating-current  motors, 
which  do  not  require  a  field  excited  by  a  continuous  current. 
These  motors,  on  the  other  hand,  take  a  considerable  amount  of 
wattless  current.  If  a  motor  of  this  kind  consumes  2000  effective 
watts,  its  apparent  watt  consumption  might  be  as  much  as  3000. 
The  generator  has  then  to  be  designed  for  an  output  of  3000  watts, 
and  likewise  the  mains  have  to  be  calculated  for  a  larger  current, 
much  of  which  is  useless  for  producing  power. 

Now,  with  a  synchronous  motor,  the  magnetization  of  which  is 
effected  separately  by  continuous  current,  there  is  no  phase-difference 
as  long  as  the  excitation  is  correctly  adjusted.  Before  switching  the 
motor  on  the  mains  it  is  brought  to  the  same  periodicity,  voltage, 
and  phase  as  the  alternating  current  with  which  it  is  supplied,  and 
therefore,  after  the  motor  is  switched  on  the  mains,  there  is  no 
magnetizing  or  wattless  current  flowing  into  the  motor,  the  current 
thus  being  in  phase  with  the  voltage.  If  the  motor  consumes 
20  amps,  at  100  volts,  there  are  2000  watts  used. 

If,  on  connecting  the  motor  to  the  mains,  the  excitation  is  too 
weak,  so  that  its  voltage  is  lower  than  that  of  the  alternating  current 
supplied,  then  here  a  wattless  current  would  appear,  since  the  missing 
magnetization  has,  as  it  were,  to  be  supplied  from  an  external  source. 
A  wattless  current,  and  therefore  a  phase-difference,  also  appears 
when  the  magnetization  of  the  motor  is  too  strong. 

It  is  easy  to  construct  a  vector  diagram  of  the  various  values  of 
resistance,  induction,  and  E.M.F. 7s  of  a  synchronous  motor  which  will 
illustrate  why  varying  its  field  gives  varying  phase  relation  to  its 
incoming  current.  The  E.M.F.'s  in  a  synchronous  motor  are,  first, 
the  IR  drop;  second,  the  inductance  drop,  which  combine  together 
to  give  the  impedance  drop  in  the  armature;  third,  the  E.M.F.  applied; 
and,  fourth,  the  E.M.F.  created  by  the  revolution  of  the  armature  in 


ALTERNATING-CURRENT   MOTORS 


273 


the  field,  that  is,  back  E.M.F.  All  these  values  are  out  of  phase 
with  each  other,  but  since  all  forces  must  balance  with  equilibrium, 
they  must  form  a  closed  triangle. 

In  Fig.  270,  let  o-b  equal  the  current  flowing  into  the  synchronous 
motor.  The  o-a  equals  the  IR  volts  consumed  by  resistance,  and 
o-c  equals  the  inductance  volts  consumed  by  induction.  These  two 
combine  into  o-d,  being  the  E.M.F.  consumed  by  impedance.  With 


o  as  a  centre,  draw  the  circle  e-g-h  with  a  radius  equalling  the  value 
of  volts  applied  to  the  rnotor.  About  d  as  a  centre,  draw  the  circle 
i-e-j,  intersecting  the  other  circle  at  e.  Connect  e  with  o  and  d. 
Then  the  triangle  e-o-d  contains  all  the  voltages  in  a  synchronous 
motor.  Draw  o-f  from  o  parallel  and  equal  to  d-e.  The  o-e  equals 
in  value  and  in  phase  the  applied  E.M.F.  o-b  equals  as  drawn  the 
current,  and  o-f  equals  d-e,  equals  the  back  E.M.F.  in  value  and  phase 
due  to  revolution  of  the  armature.  From  this  figure  and  with  the 
value  of  back  E.M.F.,  the  current  o-b  leads  the  E.M.F.  applied  to  the 
motor  by  the  angle  e-o-b.  If  now  the  back  E.M.F.  of  the  motor  d-e 
equals  o-f  be  made  smaller,  it  will  be  noticed  that  the  current  now 
lags  behind  the  applied  E.M.F.  Fig.  271  illustrates  this. 

Thus,  as  stated,  the  synchronous  motor  has,  by  means  of  field 
excitation  control,  the  means  to  alter  the  phase  of  the  current  entering 
it.  This  holds  true,  of  course,  whether  the  synchronous  motor  is 
single-phase  or  polyphase.  Figs.  270  and  271  can  be  regarded  as  one 
phase  of  a  polyphase  machine.  A  single-phase  synchronous  motor 
has  no  tendency  to  start,  but  a  quarter-phase  or  a  three-phase  machine 
starts  from  rest  with  a  considerable  torque  and  will  soon  carry  quite 
a  load.  This  is  done  by  the  reaction  of  the  current  induced  in  the 


274 


ELECTRICAL  ENGINEERING 


pole-pieces  and  the  field  producing  these  currents.      By  Lenz's  law 
the  armature  tends  to  move  in  such  a  direction  to  prevent  the  induc- 


.  271. 


tion  of  the  currents  causing  the  motion.     To  add  to  this  effect,  poly- 
phase synchronous  motors  have  wound  into  the  pole-pieces  a  regular 


winding,  which  acts  just  like  a  " squirrel-cage"  winding  in  the  rotor 
of  an  induction  motor.  Single-phase  synchronous  motors  are  rarely 
used.  Almost  always  three-phase  motors  are  used,  embodying  the 
advantages  of  a  fair  starting  torque,  less  pole-piece  losses,  and  tech- 
nical designing  features  better  than  the  single-phase  arrangement. 


ALTERNATING-CURRENT  MOTORS  275 

Synchronous  motors  are  particularly  useful  for  large  units.  The 
largest  alternating  motor  in  the  United  States  to-day  is  a  synchronous 
motor.  It  delivers  9000  H.P.  This  feature  of  the  synchronous 
motor  that  at  will  by  simple  field  control  the  phase  of  the  incoming 
current  can  be  controlled  sometimes  results  on  transmission  circuits 
in  the  use  of  a  motor  running  " light"  solely  for  this  purpose.  A 
plot  at  no  load  of  the  variation  of  incoming  current  with  field  strength 
is  shown  in  Fig.  272. 

The  curve  c-d-e  represents  the  plot  of  current.  As  may  be  noted, 
at  a  field  current  of  value  o-g,  the  armature  current  is  a  minimum 
at  d.  If  the  field  current  is  reduced,  the  armature  current  commences 
to  rise  until  with  field  current  o-f  it  reaches  the  full-load  current  value 
a-b.  Here  the  incoming  current  is  lagging.  If  the  field  current  is 
now  increased,  the  incoming  armature  current  commences  to  fall  till 
it  reaches  its  minimum  at  d.  Further  increase  of  current  causes  the 
armature  current  to  increase  till  full-load  current  is  again  reached, 
but  in  this  case  on  the  leading  side.  The  current  taken  at  g  is  only 
that  necessary  to  supply  the  losses  of  the  synchronous  motor  running 
light,  and  is  thus  small  in  value. 

The  synchronous  motor  must  have  its  field  circuit  excited  by 
direct  current.  For  this  purpose  a  small  direct-current  exciter  is 
belted  or  direct  connected  to  the  main  motor.  Since  on  starting 
there  is  no  field  required  on  the  synchronous  motor,  the  exciter  need 
deliver  no  current  to  the  field  of  the  synchronous  motor  till  it  reaches 
full  speed,  which  therefore  makes  feasible  the  method  of  operating 
the  exciter  from  the  synchronous  motor  field.  Thus,  even  though 
direct  current  is  necessary,  the  unit  is  self-contained,  requiring  only 
itself  and  the  alternating  energy  to  do  its  work.  At  starting  with 
the  armature  stationary,  the  field  spools  form  a  secondary  of  a  trans- 
former of  which  the  armature  is  the  primary,  and  since  the  field  turns 
are  high  as  compared  with  the  armature  a  voltage  is  induced  in  them 
higher  than  the  voltage  applied  to  the  armature.  Thus,  it  is  dan- 
gerous to  be  near  the  terminals  of  the  field  at  the  instant  of  starting. 
Deaths  have  occurred  from  this  cause.  To  avoid  trouble,  the  spools 
may  be  split  up  at  starting,  or  closed  on  the  exciter,  which  entirely 
annihilates  the  voltage.  The  latter  method,  however,  reduces  the 
ability  to  start  somewhat.  Usually  the  field  insulation  is  so  designed 
that  it  will  stand  the  high  voltage  induced.  These  motors  are  very 
generally  used  for  a  large  variety  of  purposes  in  the  United  States. 


276  ELECTRICAL  ENGINEERING 


The  Rotary  Converter 

For  reasons  already  known  to  us,  alternating  currents  are  very 
frequently  employed  for  transmission  of  electrical  energy.  Now, 
there  are  many  purposes  for  which  alternating  currents  are  in- 
applicable. They  cannot  be  used  for  charging  secondary  batteries. 
At  alternating- current  central  stations  it  is  therefore  necessary, 
even  when  there  is  a  very  small  load  during  the  daytime,  to  have 
one  or  more  generators  running.  Also  the  valuable  "buffer  effect" 
of  secondary  batteries  cannot  be  used  in  alternating-current  central 
stations.  To  combine  the  advantages  of  alternating  currents  with 
those  of  continuous  currents,  the  following  scheme  is  employed  in 
many  cases  for  transmission  of  energy  to  long  distances: — In  the 
central  station  alternating  current  is  produced  and  is  led  to  a 
number  of  sub-stations  distributed  over  the  area  of  supply.  In  these 
sub-stations  the  alternating  current  is  transformed  into  continuous 
current,  and  at  the  sub-station  secondary  batteries  are  generally 
employed.  For  certain  hours  the  secondary  batteries  in  the  sub- 
stations are  charged,  thus  providing  current  for  the  time  of  small 
demand  when  the  machines  in  the  central  station  as  well  as  in  the 
sub-stations  are  shut  down. 

At  a  sub-station,  machines  are  required  for  transforming  alternat- 
ing into  continuous  current.  For  this  purpose  either  two  separate 
machines,  viz.  one  alternating-current  motor  coupled  directly  to  a 
continuous-current  generator,  which  combination  is  generally  called 
a  motor  generator,  or  a  single  machine,  with  a  rotating  armature, 
may  be  employed,  having  slip-rings  on  one  side  and  a  commutator 
on  the  other.  A  machine  of  the  latter  type  is  generally  called  a 
converter. 

In  both  cases  synchronous  motors  can  be  used  without  any 
disadvantage,  for  the  secondary  battery  installed  at  the  sub-stations 
will  serve  for  exciting  the  synchronous  motors.  The  procedure 
is  quite  simple.  In  the  case  of  the  motor  generator  the  con- 
tinuous-current generator  is  started  as  a  motor  by  means  of  the 
secondary  battery  and  its  speed  regulated  until  it  is  that  required 
for  synchronism.  Then  the  synchronous  motor  is  excited  and 
the  switch  closed.  The  synchronous  motor  now  drives  the  con- 
tinuous-current machine,  and,  by  more  strongly  exciting  the  latter, 
its  E.M.F.  increases  above  that  of  the  battery,  so  that  the  con- 
tinuous-current machine  supplies  current  to  the  battery;  i.e.,  it  is 
working  as  a  generator.  Each  of  the  two  coupled  machines  may 
be  built  for  any  voltage.  For  example,  the  synchronous  motor 


THE  ROTARY  CONVERTER  277 

might  be  built  for  a  voltage  of  2000  or  5000,  and  the  continuous- 
current  dynamo  for  110,  220,  500,  or  any  other  voltage. 

The  synchronous  motor  can  also  be  started  by  itself,  as  has  been 
explained.  Under  these  conditions  there  is  a  large  drawing  of  current 
at  low-power  factor  (say  double  the  normal  operating  current),  so 
that  the  voltage  upon  the  line  is  affected  considerably.  If  this  is 
troublesome,  the  starting  from  batteries  by  synchronizing  can  be 
done,  which  cuts  out  all  the  trouble.  Another  method  of  starting 
synchronous  motor  generator  sets  is  to  use  a  compensator,  so  that 
just  the  required  amount  of  current  is  given  to  the  synchronous 
motor,  the  line  current  being  reduced  in  proportion  to  the  ratio  of 
the  compensator.  After  the  motor  is  well  started,  throw  one  switch 
within  the  compensator  (or  without),  which  gives  normal  voltage 
again  to  the  motor.  Since  the  maximum  current  occurs  with  the 
armature  at  rest,  sometimes  the  motor  is  given  a  start  by  mechanical 
means  provided,  such  as  a  rod  inserted  in  holes  in  the  shaft. 

With  a  converter  the  case  is  different.  It  is  impossible  to  use 
it  for  direct  transformation  of  high-tension  alternating  into  low- 
tension  continuous  current. 

Any  alternating-current  dynamo  provided  with  a  commutator  and 
slip-rings  like  that  shown  in  Fig.  242  can  be  used  as  a  converter. 
The  armature  of  the  converter  can  have  either  a  single  winding 
connected  with  slip-rings  and  commutator  or  two  separate  windings. 
In  this  latter  case  one  of  them  has  to  be  connected  with  slip-rings, 
the  other  with  the  commutator. 

Both  the  motor  generator  and  converter  may  be  used  for  many 
different  purposes.  They  can  be  used  as  (1)  a  continuous-current 
motor,  (2)  continuous-current  generator,  (3)  a  synchronous  motor, 
(4)  an  alternator,  (5)  a  dynamo  for  continuous  and  alternating  currents 
simultaneously,  (6)  a  continuous-current  to  alternating-current  trans- 
former, and  (7)  finally  an  alternating-current  to  continuous-current 
transformer. 

Since  in  all  these  cases  of  the  use  of  a  converter  continuous  and 
alternating  currents  are  either  produced  or  transformed  in  one 
armature,  it  is  clear  that  there  must  exist  a  definite  proportion 
between  the  continuous  and  alternating  voltage,  and  that,  unlike  the 
motor  generator,  it  is  impossible  with  the  converter  to  transform 
alternating  current  into  continuous  current  of  any  voltage.  The 
ratio  between  the  two  voltages  may  be  determined  by  the  help  of 
a  simple  consideration.  We  shall  first  of  all  consider  an  armature 
with  a  single  winding. 

In  dealing  with  the  Gramme  ring,  as  an  alternating-current 
armature  (see  p.  247),  we  learned  that  the  maximum  alternating 
voltage  is  produced  if  the  windings  connected  with  the  slip-rings 
are  just  in  the  neutral  zone.  Now,  this  is  the  normal  voltage  of  the 
continuous  current  produced  by  the  same  ring,  since  in  this  case 


278  ELECTRICAL  ENGINEERING 

the  brushes  are  always  in  the  neutral  zone.  Thus  we  have  the  simple 
equation: — In  a  single-phase  converter  maximum  alternating-current 
voltage  is  equal  to  the  normal  continuous-current  voltage.  We  have 
learned  that  the  measured  or  effective  value  of  the  alternating- 
current  voltage  is  equal  to  about  0.7  of  its  maximum  voltage.  Hence, 
if  with  this  converter  a  continuous  voltage  of  100  is  produced,  then 
the  effective  voltage  of  the  alternating  current  taken  from  the  slip- 
rings  will  be  about  70. 

Owing  to  the  ohmic  loss  in  the  armature  wires,  the  secondary 
voltage  of  a  rotary  converter  will  be  somewhat  smaller  than  that 
found  by  the  above  calculation.  If  the  machine  be  used  as  a  con- 
tinuous- to  alternating-current  converter,  we  get,  at  a  continuous 
voltage  of  100,  not  quite  70  volts  on  the  alternating-current  side, 
but,  according  to  the  load  of  the  machine,  somewhat  less— say  69, 
perhaps  68,  volts  only.  If,  on  the  other  hand,  we  use  the  machine 
as  an  alternating-  to  continuous-current  converter,  we  shall  for  70 
volts  alternating  current  get  less  than  100  volts  continuous  current, 
perhaps  only  98  or  97  volts.  If  there  be  two  separate  windings  on 
the  armature,  the  winding  connected  with  the  slip-rings  having 
three  times  as  many  turns  as  that  of  the  winding  that  is  connected  to 
the  commutator,  then  to  a  continuous  current  of  100  volts  an  alter- 
nating current  of  3X70  =  210  volts  would  correspond.  In  any  case 
there  exists  a  definite  relation  between  alternating  and  continuous 
voltage  which  cannot  be  altered  by  the  regulation  of  the  continuous- 
current  excitation.  If  for  charging  cells  we  want  to  increase  the 
continuous  voltage  from  100  to  150,  then  we  must  increase  the  voltage 
of  the  alternating  current  supplied  to  the  slip  ring  by  one-half. 

Since  to  the  rotating  armature  of  a  converter  alternating  current 
has  to  be  supplied,  it  is  impossible  to  employ  machines  of  this  kind 
for  directly  converting  high-tension  alternating  into  low-tension  con- 
tinuous current.  For  this  purpose  a  further  apparatus,  an  ordinary 
or  static  transformer,  is  required,  which  first  transforms  the  high- 
tension  alternating  current  of,  say,  2000  volts  into  a  low-tension 
alternating  current  of,  say,  70  volts.  This  alternating  current  may 
then,  by  a  rotary  converter,  be  converted  into  a  continuous  current 
of  about  100  volts. 

A  converter  is  started  in  the  same  way  as  a  motor  generator. 
The  machine  is  first  excited,  then  started  as  a  continuous-current 
motor,  and,  as  soon  as  it  is  running  in  synchronism,  it  is  switched 
on  the  alternating-current  circuit. 

Often  converters  are  started  from  the  A.C.  end  when  they  are 
not  single-phase.  As  a  matter  of  fact,  single-phase  converters  are 
rarely  used  in  the  United  States.  Three-phase  converters  are  almost 
universally  used.  Like  the  polyphase  synchronous  motor,  a  three- 
phase  converter  will  start  from  its  own  A.C.  current.  About  30  to 
40  per  cent,  normal  voltage  is  required.  When  this  is  applied  the 


THE  ROTARY  CONVERTER 


279 


rotary  will  quickly  come  up  to  speed,  drawing  from  the  line  about 
double  current.  There  is  practically  no  sparking  at  the  D.C.  brushes 
under  these  conditions.  Sixty-cycle  rotaries,  due  to  their  very  small 
armature  reaction,  draw  more  current  from  the  line  than  do  25-cycle. 
The  ratio  of  A.C.  to  D.C.  voltage  in  a  three-phase  rotary  is  different 
than  that  of  a  single-phase.  Consider  Fig.  273. 


FIG.  273. 


Let  the  letters  a,  b,  c  represent  the  points  where  the  A.C.  taps 
are  connected  to  the  winding,  for,  as  has  been  stated,  a  three-phase 
rotary  converter  consists  simply  of  a  D.C.  generator  (with  commu- 
tator and  brushes)  having  taps  in  its  winding  at  three  equidistant 
points  which  are  connected  to  three  collector-rings.  Into  these  col- 
lector-rings three-phase  current  is  given,  and  out  of  the  commutator 
direct  current  is  taken. 

Let  the  armature  be  in  the  position  shown.  The  D.C.  brushes, 
being  at  b  and  e,  b-e  equals  the  direct-current  voltage  and  the  maxi- 
mum A.C.  voltage  of  a  single-phase  converter.  Thus  b-d  equals  one- 
half  the  D.C.  voltage.  In  the  triangle  b-d-a,  the  value  b-d  equals 
d-a  is  thus  known,  as  well  as  the  angle  b-d-a,  and  the  angles  a-b-d 
and  d-a-b  are  equal.  Thus  the  line  b-a  can  be  found,  but  b-a  rep- 
resents the  three-phase  voltage  of  the  converter;  i.e.,  the  voltage 

between  collector-rings,     b-a  equals  \/3Xb-d=— —Xb-e.    But  b-e 
equals  the  maximum  of  the  single-phase  voltage.     Thus,  the  virtual 

E.M.F.,  or  the  square  root  of  mean  square  voltage,  b-a= X— /=-» 

2      V2 

since  the  ratio  of  maximum  to  virtual  equals \/2,  as  has  been  shown. 
Calling  voltage  6-e=E  the  D.C.  voltage,  we  get  the  A.C.  voltage' 
between  collector-rings  (equal  a-b,  Fig.  273)  equals  the  D.C.  voltage 

v/o 
E  multiplied  by^— ^.  =  0.612E.     Assuming   the   converter   to   be  of 


280  ELECTRICAL  ENGINEERING 

100  per  cent,  efficiency,  the  input  equals  the  output.  _In  a  three- 
phase  circuit  the  input  is,  as  will  be  shown  later,  ET\/3.  The  D.C. 
output  is,  as  has  been  shown  previously,  IE  when  E'  equals  the 
alternating  E.M.F.  between  collector-rings,  V  the  current  in  the  line 
to  the  collector-rings,  and  E  and  I  the  D.C.,  E.M.K,  and  current. 


Thus,  E'lV3"=EI.     But  E'  =  EX  Thus,  =  El,  or 


o 

Since  the  efficiency  is  not  100,  but  nearer  94,  the  current  I'  in  the 
A.C.  line  has  not  only  to  supply  the  output  but  the  losses.  This  1' 
is  about  6  per  cent,  more  than  the  above,  or  about  equal  to  the  D.C. 
current.  Thus,  in  a  three-phase  converter  the  A.C.  and  D.C.  currents 
are  about  alike.  Since  both  the  A.C.  currents  and  the  D.C.  current 
flow  in  the  same  wires  in  the  armature,  and  since  under  such  con- 
ditions there  cannot  be  two  separate  currents  actually,  it  follows 
that  they  must  combine.  Since  also  the  A.C.  currents  act  as  driving 
power  and  the  D.C.  as  energy  given  out,  it  follows  that  these  two 
currents  tend  to  flow  opposite  in  direction  and  thus  tend  to  neutralize 
each  other.  We  thus  have  in  the  windings  of  a  rotary  converter 
D.C.  and  A.C.  currents  in  opposition.  It  can  be  expected  that  since 
one  current  has  a  sine  wave  in  shape  and  the  other  a  steady  value 
that  this  combination  is  rather  complicated.  Without  covering  the 
matter  in  detail,  it  has  been  found  that  the  resulting  current  in  a 
three-phase  converter,  when  squared  (this  representing  the  heat 
produced  in  the  windings)  is  58  J  per  cent,  of  the  square  of  the  D.C. 
current.  This  value  allows  for  the  efficiency  of  the  converter.  From 
this  it  can  be  at  once  seen  that  a  rotary  of  a  given  size  will  heat  less 
than  a  D.C.  machine  of  the  same  size,  and  thus  a  rotary  is  smaller 
for  the  same  heating  and  therefore  cheaper  than  a  D.C.  machine, 
which  is  true.  In  addition  to  this,  it  is  apparent  that  since  the  A.C. 
current  flows  in  one  direction  and  the  D.C.  in  the  other,  that  there 
is  no  armature  reaction,  and  thus  no  brush  shift  is  required  with 
change  of  load.  Thus,  a  rotary  must  be  better  in  commutating  char- 
acteristics than  an  ordinary  D.C.  machine.  As  a  matter  of  fact, 
rotaries  require  no  shift  of  brushes  and  will  carry  three  times  normal 
load  without  difficulty.  They  are  thus  especially  suitable  for  railway 
lines  when  excessive  load  may  momentarily  come  on. 

Since  the  A.C.  end  of  a  rotary  acts  just  like  a  synchronous  motor, 
it  naturally  follows  that  the  phase  of  the  entering  current  can  be 
altered  by  altering  the  field  strength,  a  leading  current  resulting 
from  strengthening  the  field  and  a  lagging  from  weakening  it.  Ad- 
vantage is  taken  of  this  in  rotaries  to  regulate  the  D.C.  voltage,  A 
series  field  is  placed  on  the  rotary,  and  as  the  D.C.  load  comes  on 
the  field  is  strengthened.  As  it  strengthens  the  A.C.  current  comes 


COMMUTATOR   MOTORS  281 

more  and  more  leading,  holding  up  the  voltage.  To  increase  the 
effect,  inductance  is  inserted  in  the  A.C.  lines,  and  since  A.C.  current 
in  passing  through  inductance  raises  the  voltage  if  the  current  is 
leading,  a  combination  of  inductance  and  field  strength  may  be 
chosen,  so  that  a  constant  or  rising  D.C.  voltage  will  result.  Thus, 
rotaries  can  over-compound  on  their  D.C.  ends  just  as  ordinary  D.C. 
machines. 

Rotaries  are  extensively  used  in  the  United  States  for  sub-stations 
to  supply  lights  or  power.  They  are  low  in  cost  per  kilowatt  and 
capable  of  large  overloads  and  in  general  are  very  important  adjuncts, 
in  electrical  distribution  of  power. 


Commutator  Motors 

The  question  may  be  asked,  Is  it  possible  to  run  a  continuous- 
current  motor  with  alternating  current? 

We  are  acquainted  with  the  fact  that  the  direction  of  rotation 
of  a  continuous-current  motor  remains  the  same  if  we  change  the 
mains  leading  to  the  motor  (p.  145),  for  the  reason  that  both  the 
magnet  field  and  the  armature  current  change  their  direction.  It 
must  hence  follow  that  we  are  able  to  get  motive  power  from  a, 
continuous-current  motor  supplied  with  an  alternating  current- 
Naturally  the  magnet  system  of  the  motor  must  not  be  solid,  but 
must,  like  all  cores  of  alternating-current  magnets,  consist  of  insulated 
iron  disks.  Otherwise  its  construction  is  quite  similar  to  an  ordinary 
continuous-current  motor.  Commutator  motors  are  generally  built 
as  series  motors. 

Let  us  now  consider  the  starting  of  the  motor.  The  motor  has 
to  be  switched  on  the  alternating-current  mains.  Armature  and 
magnet  coils  are  then  traversed  by  the  same  current.  The  armature 
wires  in  the  magnetic  field  tend  now  to  turn  the  armature  in  a  definite 
direction — say,  for  instance,  clockwise.  The  armature  is  therefore 
turned  a  little,  but  before  it  has  turned  through  one  revolution  the 
direction  of  the  armature  current  is  altered.  At  the  same  instant 
the  direction  of  the  magnet  current  is  also  altered.  The  effect  after 
the  change  of  the  current  direction  is  the  same  as  it  was  before;  i.e., 
the  armature  is  turned  again  clockwise,  and  thus  the  motor  will  start. 
Since,  however,  the  armature  windings  short-circuited  by  the  brushes 
are  traversed  first  by  a  negative,  then  by  a  positive  current,  these 
motors,  on  starting,  violently  spark,  and  sparkless  running  is  difficult 
or  impossible  to  obtain. 

Alternating  D.C.  motors  have  characteristics  similar  to  D.C. 
motors,  differing  only  in  this  fact,  that  the  current  lags  behind  the 
applied  E.M.F.  to  the  motor,  which  condition  cannot,  of  course,  apply 


282  ELECTRICAL  ENGINEERING 

to  D.C.  motors.  Thus,  the  line  drop  in  the  transmission  is  greater 
than  with  D.C.  motors,  since,  as  has  been  shown,  the  line  drop  is 
greater  the  greater  the  lag  of  current  for  a  given  condition  of  the 
line.  Also  the  generator  must  be  large  to  furnish  this  lagging  current 
and  must  be  better  in  regulation.  In  spite  of  the  sparking  tendency, 
which  is  excessive  at  starting,  this  type  of  motor  has  been  introduced, 
and  roads  are  now  operating  using  them.  In  order  to  reduce  the 
sparking  resulting  from  the  pulsating  flux  through  the  armature  coil 
short-circuited  by  the  brushes,  the  leads  to  the  commutator  are  made 
high  in  resistance,  increasing  the  resistance  of  the  circuit  in  which 
the  short  circuit  acts.  By  this  means  the  motors  are  made  operative. 
More  attention  must  be  given,  however,  to  the  commutator  to  keep 
it  in  good  running  condition.  On  railway  lines  a  good  deal  of  coasting 
is  done  by  the  cars,  during  which  time  no  current  is  flowing  into  the 
motor.  During  this  time  the  commutator  gets  polished  up  by  the 
brushes,  partly  or  wholly,  depending  upon  the  condition  of  the  injury 
done  by  the  sparking  when  current  is  flowing  into  the  motors. 


CHAPTER  XI 


MULTIPHASE  ALTERNATING  CURRENT 


Induction   Motors— Rotating   Field 

NEITHER  of  the  two  alternating-current  motors  described  in  the 
last  chapter  is  so  simple  in  some  respects  as  the  continuous-current 
motor.  Whilst  the  alternating-current  generator  and  transformer 
are  far  simpler  than  the  corresponding  continuous-current  appliances, 
with  the  motor  the  contrary  would  seem  to  be  the  case. 

It  is  important  now  to  point  out  that  we  can,  with  alternating 
currents,  produce  motion  by  availing  ourselves  of  the  effects  of 

induction.  We  have  seen  this  with  a 
metal  ring  which  was  repelled  by  an  alter- 
nating current  flowing  through  an  electro- 
magnet. On  switching  the  coil  in  circuit 
the  ring  was  pushed  upwards,  on  stopping 
the  current  the  ring  fell  down. 

An  up  and  down  motion  of  this  kind 
is  insufficient  for  a  motor.  What  we 
want  is  a  means  of  producing  rotating 
motion. 

The  Italian  electrician  Ferraris  found 
that  by  two  alternating  currents  differing  in 
phase  a  rotating  field  can  be  produced. 
Fig.  274  shows  two  coils,  A  and  B,  whose 
windings  are  at  right  angles  to  each  other. 
These  coils  are  traversed  by  alternating 
currents  which  differ  in  phase  by  90°. 
Either  of  these  coils  in  itself  would  produce  a  pulsating  field,  but 
the  two  coils  together  produce  a  rotating  field. 

A  simple  experiment  with  a  freely  suspended  stick,  or,  still  better, 
a  stone  suspended  by  a  string,  gives  us  a  corresponding  example,  and 
will  make  the  matter  clear.  If  we  push  such  a  pendulum  from 
its  position  of  rest,  then  it  will  swing  to  and  fro.  A  complete 

283 


X 

FIG.  274.—  Production  of 
Rotating  Field. 


284  ELECTRICAL  ENGINEERING 

movement  from,  say,  the  left  to  the  right  and  back  to  the  left  is 
called  a  period.  If,  from  its  position  of  rest,  we  push  the  stone 
from  us,  it  will  then  take  up  a  swinging  motion  from  front  to  back, 
which  differs  from  the  first  vibration  in  direction  only,  but  not  in  the 
kind  of  motion.  If,  now,  we  push  the  pendulum,  firstly,  from  its 
position  of  rest  towards  the  right;  and,  secondly,  after  a  quarter- 
period — that  is,  after  it  has  made  half  an  oscillation,  being,  therefore, 
in  its  extreme  position  to  the  right — we  push  it  forwards,  we  shall 
observe  that  the  pendulum  takes  up  a  rotating  motion.  It  swings 
no  longer  in  a  single  plane,  but  in  a  circle.  The  motion  in  a  straight 
line  has  been  changed  into  a  rotating  motion. 

It  is  essential  for  the  second  impulse  to  take  place  in  a  direction 
which  is  at  right  angles  to  the  first  impulse,  and  also  that  the  time 
when  the  second  impulse  takes  place  is  a  quarter  of  a  period  later 
than  that  of  the  first  impulse.  If,  whilst  the  pendulum  is  swinging 
from  left  to  right,  we  strike  it  in  the  direction  from  front  to  back 
just  at  the  instant  it  passes  its  lowest  position,  we  do  not  now  get 
a  rotating  motion  of  the  pendulum,  but  it  will  swing  in  a  direction 
between  the  directions  of  the  two  impulses. 

Similarly  the  two  coils  in  Fig.  274,  each  of  which  alone  is  capable 
of  producing  a  pulsating  field,  are  able  to  set  up  a  rotating  field, 
provided  that  they  are  traversed  by  two 
alternating  currents,  the  phase-difference 
between  which  is  a  quarter-period.  We 
know  that  the  direction  of  a  field  is 
that  indicated  by  a  freely  movable  north 

pole.     Let  us  now  imagine  a  north  pole     ^- 

under  the  influence  of  coil  A  (in  Fig.  275     [BJ 

the    coils    are    shown    more    distinctly    in  

cross-section).    The  coil  A  tends  to  drive 

the  north  pole  at  right  angles  to  its  plane 

from  left  to  right — that  is;  in  the  direction 

of  the  single-barbed   arrow.    Coil  B  alone 

will    try    to    drive    the    pole    from    front  FIG.  275. 

to    back    in    the    direction    of   the   arrow 

with  two  barbs.     If,  now,  the  currents  differ  by  a  quarter-period, 

exactly  the  same  will  take  place  as  with  the  pendulum.     The  pole 

will  rotate. 

What  influence  will  this  rotating  field  exert  on  a  metal  cylinder,  C, 
suspended  in  its  interior? 

A  magnetic  field  rotating  about  a  conductor  produces,  as  we 
know,  in  the  latter  an  E.M.F.,  and  if  there  is  a  closed  circuit,  electric 
currents  result.  These  currents  have  their  direction  so  as  to  resist 
any  motion,  following  Lenz's  law.  In  the  metal  cylinder  C,  in  Fig. 
274,  such  currents  will  be  produced.  These  currents,  first,  tend  to 
weaken  the  primary  field  (just  as  the  currents  in  the  secondary  coil 


MULTIPHASE  ALTERNATING  CURRENT  285 

of  a  transformer  do),  and,  secondly,  they  resist  the  motion  of  the 
field,  which  will  rotate  as  long  as  the  primary  currents  differ  in  phase 
by  a  quarter-period.  The  metal  cylinder  within  this  field  will  be 
acted  upon  in  a  certain  way.  Consider  for  a  moment  what  happens 
to  any  one  who  tries  to  stop  a  heavy  and  fast-moving  carriage  by 
taking  hold  of  it.  The  attempt  will  be  a  failure,  for  he  will  be  carried 
along  with  the  vehicle.  In  the  same  way,  the  armature  C,  which  re- 
sists the  rotating  motion  of  the  field  without  being  able  to  stop  it, 
will  be  taken  with  the  rotating  field,  i.e.,  it  will  be  turned  round  its 
axis. 

Hence  there  will  be  a  tendency  to  turn  the  armature  with  the 
same  speed  as  that  with  which  the  field  is  rotating.  This  state  can, 
however,  never  be  perfectly  reached,  for  if  the  armature  ran  in 
synchronism  with  the  field,  the  effect  on  the  armature  would  be  the 
same  as  if  field  and  armature  were  at  rest,  and  no  current  could  be 
induced  in  the  armature.  The  result  will  be  that  the  armature 
can  now  no  longer  exert  any  force,  and  it  will  slow  up  owing  to  the 
frictional  resistances.  As  soon  as  this  happens,  the  armature  is  again 
crossed  by  lines  of  force,  a  current  is  again  induced  inside,  it  exerts 
a  force,  and  thus  is  able  to  overcome  the  frictional  resistances.  The 
greater  the  load  becomes,  the  slower  the  armature  will  run  in 
comparison  with  the  speed  of  the  rotating  field.  The  consequence 
will  be  that  stronger  currents  are  induced  in  the  armature,  enabling 
it  to  overcome  the  heavier  load.  The  armature  currents  have  a 
further  important  action — they  also  tend  to  weaken  the  primary 
field,  and  this  will  now,  just  in  the  same  way  as  the  primary 
coil  of  a  transformer,  take  more  current  when  it  is  connected  with  a 
source  of  constant  voltage.  Hence  we  observe  that  the  behaviour  of 
this  kind  of  motor  is  very  similar  to  that  of  continuous-current  shunt 
motors. 

Motors  depending  on  this  principle  are  called  induction  motors, 
or  asynchronous  motors.  They  are  called  asynchronous  because 
their  working  principle  depends  on  the  fact  that  they  do  not  run 
synchronously;  but  their  speed  is  less  than  the  speed  of  synchronism. 

The  amount  the  armature  speed  of  an  asynchronous  motor  is 
less  than  the  speed  of  rotation  of  the  field  is  called  the  "slip." 

We  shall  now  deal  with  the  construction  of  a  2-phase  induction 
motor.  To  obtain  sufficiently  strong  magnetic  fields ,  both  the  outer  a  nd 
inner  parts  have  to  be  built  up  from  the  iron  disks,  and  the  windings 
have  to  be  laid  in  slots.  We  have  here  two  circular  parts,  the  cores 
of  which  are  built  up  like  that  of  a  continuous-current  armature. 
In  its  simplest  form  (see  Fig.  276)  the  outer  stationary  armature, 
called  the  "primary  armature"  or  "stator,"  has  four  slots.  Into 
every  two  opposite  slots,  AA  and  BB,  the  coils  are  laid  which  cor- 
respond to  the  first  and  second  phase  respectively.  On  the  cir- 
cumference of  the  inner,  rotating  armature,  or  so-called  rotor, 
there  are  a  number  of  slots  or  holes  through  which  wires  are  drawn. 


286 


ELECTRICAL  ENGINEERING 


These  wires  can  be  connected  with  each  other  in  many  different  ways. 
One  method  of  connection  is  shown  in  Fig.  277,  which  represents  the 
type  called  a  squirrel-cage  rotor.  At  the  front  and  the  back  of  the 
armature  all  the  wires  are  connected  by  copper  rings. 

If  on  the  stator  there  were  the  coil  A  only,  and  this  coil  were 
traversed  by  a  continuous  current  in  the  direction  marked  in  Fig.  278 
by  a  cross  Lnd  dot  respectively,  it  would  produce  a  magnetic  field  as 
shown  in  this  figure.  The  lines  of  force  leave  the  left  part  of  the 
stator  and  enter  the  right  part,  making  the  former  a  north  and  the  latter 
a  south  pole.  If  the  coil  has  an  alternating  current  passing  through  it, 
then  at  a  certain  instant  the  left  part  will  have  the  strongest  north, 


FIG.   276.— Two-phase  Motor. 


FIG.    277. — Squirrel  Cage. 


and  the  right  part  the  strongest  south,  magnetism.  The  magnetism 
will  then  gradually  become  weaker,  until  it  is  reduced  to  nothing,  then 
it  will  be  reversed,  and  so  on.  Similarly  coil  B  alone,  if  traversed  by 
a  continuous  current  in  the  direction  marked,  would  cause  the  lower 
part  of  the  outer  armature  to  become  a  north,  and  the  upper  part  a 
south  pole  (see  Fig.  279),  whilst  with  alternating  currents  the  polarity 
would  continually  be  reversed. 

Now  coils  A  and  B  are  simultaneously  supplied  with  alternating 
currents,  which  differ  in  phase  by  a  quarter -period.  Hence,  if  the 
current  in  A  is  a  maximum,  that  in  B  will  be  a  minimum  or  nothing. 
It  is  as  if  coil  B  did  not  at  this  moment  exist.  A  only  produces 
magnetism,  say,  for  instance,  a  north  pole  on  the  left.  Now,  the 
current  in  A  decreases,  whilst  that  in  B  increases  (see  the  wave- 
lines  in  Fig.  280).  Hence  A  continues  to  produce  a  north  pole 
on  the  left,  B  tends — firstly  in  a  weak,  but  later  in  a  stronger 
manner — to  produce  a  north  pole  at  the  bottom.  Both  actions  are 
therefore  combined,  and  there  will  appear  a  north  pole  at  the  left 


MULTIPHASE  ALTERNATING  CURRENT 


287 


lower  quarter,   which  will   be  lower  in   position  the   stronger  the 
current  is  in  B,  and  the  weaker  it  is  in  A.      After  a  quarter-period 


FIG.  278. 


FIG.  279. 


the  current  in  A  becomes  zero,  whereas  the  current  in  B  is  now  a 
maximum,  and  thus  the  north  pole  is  produced  only  by  the  latter. 
The  current  in  B  now  decreases,  and  the  current  in  A  has  changed  its 
direction,  and  tends  to  produce  a  north  pole  at  the  right.  By  the  com- 


FIG.  280.  —Two-phase  (or  Quarter-phase)  Current. 


bined  action  of  the  coils  A  and  B,  the  north  pole  will  now  travel  from 
the  bottom  to  the  right;  and  again,  after  a  quarter-period,  the  north 
pole  will  be  produced  on  the  right-hand  side,  since  at  this 
moment  the  current  in  B  becomes  zero  again.  We  have  therefore 
in  the  stationary  outer  armature  a  rotating  magnetic  field,  which 
makes  a  quarter  of  a  revolution  during  each  quarter-period  of 


288 


ELECTRICAL  ENGINEERING 


FIG.  281. — Four-pole  Two-phase  Motor. 


the  current,  and  a  whole  revolution  during  each  complete  period. 
This  rotating  field  produces  currents  in  the  conductors  of  the 
squirrel-cage,  causing  it  to 
revolve. 

The  speed  of  the  rotor 
differs  but  little  from  the 
theoretical  speed  of  the  rotat- 
ing field.  If,  for  instance, 
the  current  flowing  in  the 
stator  makes  6000  alterna- 
tions— that  is,  3000  periods 
or  cycles  per  minute,  then 
the  rotor  will  make  nearly 
3000  revolutions  per  minute. 
If  the  motor  is  not  loaded, 
and  the  armature  there- 
fore has  to  overcome  only 
the  frictional  resistance  in 
the  bearings,  then  even  with 
the  most  accurate  speed- 
counters  no  difference  between  the  speed  of  the  field  and  that  of 
the  motor  can  be  measured.  On  the  other  hand,  if  the  motor  is 
loaded,  its  speed  will  fall 
down  to  about  2900,  2800, 
or  even  2700.  The  motor 
has  then  a  slip  of  100,  200, 
or  300  revolutions,  or  ex- 
pressed as  a  percentage  of 
3,  6,  or  10  per  cent. 

The  windings  considered 
have  2  poles.  Two-pole  in- 
duction motors  are  seldom 
used.  Generally  the  motors 
are,  according  to  their  size, 
wound  with  four,  six,  eight, 
or  more  poles.  The  diagram 
of  a  4-pole  2-phase  motor 
is  shown  in  Fig.  281.  The 
least  number  of  slots  re- 
quired in  this  case  is  eight. 
We  may  then  have  two  coils  FIG.  282. — Four-pole  Two-phase  Motor, 

in    each    phase,    as  shown 
in    Fig.  281,  or   four    coils 

in  each  phase,  as  shown  in  Fig.  282,  the  former  winding  corre- 
sponding to  a  consequent  pole  winding,  as  in  the  case  of  some 
direct-current  machines.  The  coils  are  wound  so  that  each  of 


MULTIPHASE  ALTERNATING  CURRENT 


289 


them  tends  to  produce  in  the  part  it  surrounds  the  same  polarity, 
say,  for  instance,  each  a  north  pole.  Then,  in  the  left-  and  right-hand 
parts,  north  poles  are  produced  by  the  coils,  marked  by  full  lines, 
and  therefore,  as  consecutive  poles,  south  poles  will  appear  in  the 


FIG.  283. — Primary  Ready  for 
Winding. 


284. — Primary  Completely 
Wound. 


upper  and  lower  quarters.  The  second  phase  (represented  by  the 
winding  which  is  marked  by  dotted  lines)  produces  also  a  4-pole 
field,  which  here  lags  behind  the  first  field  by  an  eighth  of  the  whole 


FIG.  285.— Secondary  Complete.        FIG.  286.— Type  C  Motor  Complete. 


circumference.     Thus,  each  quarter-period  of  the  current  will  corre- 
spond to  J  revolution  of  the  rotating  field.     Two  periods  of  the 


290 


ELECTRICAL  ENGINEERING 


current  correspond  to  one  revolution  of  the  field,  and  3000  periods 
to  1500  revolutions  of  the  field,  and  nearly  1500  revolutions  of  the 
armature.  A  6-pole  motor  will  run  with  a  speed  of  nearly  1000, 
an  8-pole  with  nearly  750,  and  a  12-pole  with  nearly  500  revolutions 
per  minute. 

Fig.  283  shows  the  field  of  a  2-phase  motor  ready  for  winding. 
Fig.  284  shows  the  field  completely  wound.  Fig.  285  is  the  wound 
rotor,  and  Fig.  286  is  the  complete  machine. 


Three-phase  Current 

A  rotating  field  can  also  be  produced  in  other  ways  than  by 
the  method  of  two  windings  at  right  angles  to  each  other,  and 
traversed  by  currents  with  a  phase-difference 
of  a  quarter-period.  The  most  frequent 
arrangement  employed  for  producing  rotat- 
ing fields  is  that  with  three  windings,  with 
an  angle  of  120°  between  each  other  (in  a 
2-pole  field),  when  through  these  windings 
three  alternating  currents  are  passed,  each 
of  which  has  a  phase-difference  of  one- 
third  of  a  period  with  reference  to  the 
two  other  currents. 

In  Fig.  287  the  three  coils  A,  B,  and 
C,  and  in  Fig.  288  the  courses  of  the  three  FIG.  287. 

respective  currents  a,  b,  and  c  are  shown. 

The  dotted  wave-line  6  is  one-third  of  a  period  behind  the  full  wave- 
line  a,  but  is  in  advance  by  one-third  of  a  period  of  the  wave  c, 


\  / 

\/ 

s* 

^ 

yl 

^ 

*' 

\       y 
\    / 

'' 

^N. 

\    / 
\/ 

^ 

^ 

\A 

/ 

\ 

\ 

f 

/ 

\ 

2 

/ 

\ 
\ 

3 

/ 

\ 

\ 

/ 

\ 

\ 
\ 

/ 

\ 

/ 

\ 
\ 

/ 

\ 

\ 

\ 

/' 

\ 

^'' 

--'•' 

A 

\^ 

^ 

A 

j 

/N 

^ 

^ 

FIG.  288.— Three-phase  Current. 


represented  by  a  line  made  of  up  dots  and  dashes;  hence  a  remains 
behind  c,  but  runs  before  b.     At  a  definite  moment  1  (see  Fig.  288 


MULTIPHASE  ALTERNATING  CURRENT 


291 


the  current  will  be  strongest  in  A  (see  Fig.  287),  thus  tending  to 
produce  a  north  pole  above  (south  pole  below).  In  B  and  C  currents- 
are  also  flowing,  but  these  are  not  so  strong  as  that  in  A.  These 
coils  are  arranged  so  that,  at  the  same  moment,  B  tends  to  produce 
a  north  pole  to  the  right  above,  and  C  also  a  north  pole  to  the  left 
above.  The  action  of  these  three  coils  is  represented  in  Fig.  289  by 
three  arrows  of  different  length. 

We  may  compare  this  with  a  coach  having  three  horses,  the  middle 
the  strongest,  which  pulls  straight  forward,  whereas  the  weaker  horses 
also  pull  forward,  but  at  the  same  time  towards  the  right  and  left 
respectively.  The  pulling  of  one  horse  to  the  right  and  the  other  one 
to  the  left  does  not  cause  a  deviation  of  the  coach  at  all;  yet  the 
vehicle  will  be  drawn  with  greater  force  than  if  the  middle  horse, 
although  it  is  the  strongest,  had  alone  been  in  harness.  Hence  we 
have  at  this  instant  the  strongest  north  pole  at,  the  top. 

Next,  the  action  of  the  coil  C  increases  gradually,  since,  as  we  see 
from  the  wave-line,  the  current  c  grows,  whilst  at  the  same  time  the 
currents  in  A  and  B  decrease.  After  a  one-third  period,  when  the 


FIG.  289. 


FIG.  290. 


FIG.  291. 


currents  have  the  values  as  indicated  in  Fig.  288  by  the  vertical  line  2t 
b  has  reached  its  maximum  value.  Since  it  flows  now  in  an  opposite 
direction,  it  produces  a  north  pole  in  the  direction  to  the  left,  and 
downwards  (see  Fig.  290).  Also  the  current  in  phase  A  has  changed 
its  direction,  whereas  the  current  in  phase  C  has  kept  its  direction. 
Hence  B,  which  now  predominates,  produces  a  strong  north  pole 
below  on  the  left,  whereas  A  tends  to  produce  a  pole  of  the  same 
name  right  at  the  bottom,  and,  finally,  C  makes  a  north  pole  above 
on  the  left  side.  It  again  resembles  a  vehicle  with  three  horses. 
The  north  pole  will  appear  on  the  left  below  in  the  same  strength 
as  it  was  previously  above. 

One-third  of  a  period  later  (see  Fig.  291)  the  north  pole  will  have 
wandered  towards  the  right,  and  again,  after  one-third  of  a  period, 
upwards.  Hence,  in  a  third  part  of  a  period  the  field  makes  the 
third  part  of  a  revolution,  and  in  a  complete  period  an  entire 


292 


ELECTRICAL  ENGINEERING 


FIG.  292. — Four-pole  Three-phase  Motor. 


revolution,  exactly  as  in  the  case  of  the  2-phase  rotating  field. 
The  effect  of  a  3-phase  field  is,  therefore,  equal  to  that  of  a 
2-phase  field.  The  com- 
bination of  three  currents, 
the  phases  of  which  differ 
by  a  third  of  a  period,  is 
called  a  rotary  or  three- 
phase  current. 

A  rotating  field  may 
also  be  produced  with  six 
coils,  which  are  arranged 
so  that  the  angle  be- 
tween any  two  coils  is 
60°,  and  which  are  tra- 
versed by  currents  dis- 
placed by  a  sixth  of  a 
period  from  each  other. 

As  with  2-phase  motors, 
2-pole  windings  are  very 
seldom  employed  with  3- 
phase  motors.  With  a  4- 
pole  winding  the  dis- 
tance between  two  coils 
is  naturally  not  equal  to 
the  third,  but  to  the  sixth 
part  of  the  circumference. 
The  winding  diagram  of 
a  4-pole  3-phase  motor  is 
shown  in  Fig.  292.  The 
consequent-pole  winding, 
previously  described,  is 
also  employed  here.  The 
two  opposite  coils  AA 
belonging  to  one  phase  are 
connected  with  each  other 
in  such  a  way  as  to  pro- 
duce, for  instance,  in  the 
part  enclosed  by  the  coils 
north  poles,  and  in  the 
parts  not  enclosed,  conse- 
quent south  poles. 

The  squirrel-cage  may        FIG.  293.— Four-pole  Three-phase  Motor, 
obviously  also  be  used  as  with  Three  Slots  per  Pole  and  Phase, 

a  rotor  for  3-phase  motors. 

Generally  2-  and  3-phase  motors  do  not  differ  in  their  mechanical 
construction,  but  merely  in  their  winding.     Without  considering  the 


\  \ 


MULTIPHASE  ALTERNATING  CURRENT  293 

winding  and  slotting  of  the  stator,   a  2-phase  motor   cannot    be 
distinguished  from  a  3-phase  motor. 

With  regard  to  slots  there  is  a  difference  between  2-  and  3-phase 
motors,  inasmuch  as  with  a  2-phase  motor  at  least  two,  and 
with  a  3-phase  motor  at  least  three  slots  per  pole  are  required. 
The  coils  may,  if  desired,  be  laid  into  any  larger  number  of  slots;  say, 
for  instance,  2,  3,  4,  or  more,  as  shown  in  Fig.  293. 


Actions  in  Induction  Motors— Squirrel-cage 
and  Slip-ring  Armatures 

The  induction  motor  armature  as  hitherto  described  is  distin- 
guished by  utmost  simplicity.  A  squirrel-cage  rotor  is  little  more 
than  a  number  of  wires  short-circuited  on  themselves.  No  current 
has  to  be  led  to  the  rotating  part  from  outside,  consequently  no  slip- 
rings  are  required.  This  is,  of  course,  a  great  convenience.  But 
this  type  of  armature  has  one  great  disadvantage:  after  it  has  once 
been  started  it  is  found  to  work  well,  but  in  starting  it  causes  trouble. 

If  by  closing  a  3-pole  switch  we  connect  the  stationary  winding 
of  a  3-phase  motor  with  the  mains,  then  the  armature,  whilst  at  rest, 
corresponds  to  the  secondary  winding  of  a  transformer,  although 
the  actual  construction  is  very  different  to  that  of  an  ordinary 
transformer.  Again,  the  field  does  not  pulsate  like  that  of  a 
common  transformer,  but  rotates.  This  rotating  field  produces 
electro-motive  forces  both  in  the  stationary  windings  (primary 
windings  of  the  transformer)  and  in  the  rotor  winding.  The  back 
E.M.F.  now  produced  in  the  stationary  winding  is,  like  that  of 
the  primary  coil  of  a  transformer,  nearly  equal  to  the  terminal 
voltage  supplied,  so  that  only  the  magnetizing  current  flows  in  the 
primary  winding  when  the  secondary  windings  are  not  shortr 
circuited. 

It  may  be  remarked  that  the  magnetizing  current  of  a  motor  must 
be  far  larger  than  that  of  a  transformer.  For  the  lines  of  force  do 
not  here  only  flow  through  iron  (see  Figs.  278  and  279),  but  have 
twice  to  pass  air  gaps.  Now,  although  the  space  between  rotor  and 
stator  is  kept  as  small  as  possible,  a  much  greater  number  of 
magnetizing  ampere-turns  is  required  than  is  the  case  with  a 
magnetic  flux  having  a  path  entirely  of  iron. 

On  starting  the  motor  the  field  rotates  with  full  speed  round  the 
still  stationary  armature.  Hence,  in  the  short-circuited  armature 
winding  an  excessive  current  will  be  produced,  which  reacts  on  the 


294  ELECTRICAL  ENGINEERING 

primary  field  of  the  stator  with  the  effect  of  so  weakening  it  that  a 
large  current  flows  to  the  stationary  windings  from  the  mains.  This 
lasts  only  a  short  time,  for  the  current  flowing  in  the  rotor  winding- 
causes  the  rotor  to  start  with  considerable  turning  effort,  so  that 
it  rotates  very  quickly.  The  quicker  the  rotor  runs,  the  nearer  it 
approaches  the  speed  of  the  rotating  field,  and  the  fewer  lines  of 
force  it  will  therefore  cut.  Consequently  the  E.M.F.  and  the  current 
induced  in  the  armature  decrease,  the  reaction  on  the  field  becomes 
smaller,  and  the  stator  absorbs  less  current. 

To  give  a  numerical  example,  a  motor  which  is  designed  for  a 
current  of  30  amps,  will,  if  running  at  full  speed  unloaded,  absorb  a 
current  (" no-load  current")  of  about  10  amps.  It  must  not  be 
thought  that  the  motor  really  requires  one-third  of  the  maximum 
energy  for  running  without  load.  The  phase-difference  is  (as  with 
the  unloaded  transformer)  great,  and  the  power  factor  is  equal  to 
about  0.2  or  0.3,  so  that  the  watts  taken  by  the  unloaded  motor 
are  about  TV  or  -fa  of  the  watts  taken  at  full  load. 

At  the  moment  of  starting,  the  current  will  be  large,  perhaps 
as  much  as  90  or  100  amps.,  which  is  about  three  times  the  normal 
current.  This  is  a  great  disadvantage  in  a  motor  with  a  squirrel- 
cage  armature.  With  very  small  motors  up  to  about  1  H.P., 
sometimes  even  with  rather  larger  motors,  such  a  sudden  rush  of 
current  might  be  allowable;  on  the  other  hand  it  is  clear  that  if  a 
20-H.P.  motor  requires  three  times  the  normal  current  at  starting, 
this  would  be  very  objectionable,  especially  when  lamps  are  also  on 
the  motor  mains. 

To  avoid  these  sudden  rushes  of  current,  we  must  not  short-circuit 
the  windings  of  the  rotor,  but  connect  them  with  slip-rings,  so  that 
resistance  may  be  inserted  in  the  rotor  circuit. 

With  slip-ring  motors  the  rotor  winding  is  similar  to  that  of  the 
stator.  For  a  3-phase  motor  it  may  be  2-,  3-,  or  poly-phase. 
Supposing  it  to  be  3-phase  like  the  stator  winding,  then  we  can 
connect  the  phases  either  in  star  or  in  mesh,  and  lead  the  ends  to  the 
three  slip-rings.  The  slip-rings  are  provided  with  brushes,  and  from 
them  cables  lead  to  the  three  regulating  resistances,  which  may  be 
connected  either  in  star  or  mesh,  and  generally  are  switched  in  or  out 
by  means  of  a  single  lever.  If  with  an  open  rotor  circuit  we  connect 
the  stator  winding  with  the  mains  by  using  a  3-pole  switch,  then  in 
the  rotor  winding  an  E.M.F.  is,  of  course,  induced;  but  since  the 
rotor  circuit  is  not  closed,  no  current  can  be  produced.  Hence  the 
rotor  does  not  exert  a  weakening  reaction  on  the  stator.  Through 
the  latter,  therefore,  only  the  magnetizing  current  will  flow — that  is, 
a  current  equal  to  only  |  or  \  of  the  normal  current. 

Round  the  armature  a  magnetic  field — say,  for  example,  a 
4-pole  one — is  rotating  with  a  speed  of  about  1500  revolutions  per 
minute.  The  effect  is  the  same  as  if  the  rotor  were  the  stationary 


MULTIPHASE  ALTERNATING  CURRENT 


295 


armature  of  a  generator  about  which  a  4-pole  field  rotates.  In 
the  armature  electro-motive  forces,  but  no  currents,  are  induced 
until  the  outer  circuit  is  closed.  Immediately  we  close  the  switch 
and  connect  the  external  circuit,  currents  flow  through  the  armature 
and  the  resistances,  and  these  currents  cause  the  armature  to 
rotate.  The  resistances  ought,  of  course,  to  be  so  selected  that 
the  currents  in  the  armature  and  in  the  stator  do  not  exceed  the 
normal  currents. 

When  the  armature  is  set  into  rotation,  the  E.M.F.  induced  in  it 
becomes  smaller  than  that  previously  induced  in  the  stationary  arma- 
ture, and  if  the  regulating  resistance  be  diminished  the  armature  will 
gain  speed.  We  can  then  gradually  diminish,  and  finally  short- 
circuit  the  resistance  when  the  armature  is  nearly  synchronous  with 
the  field.  Hence,  when  actually  at  work  under  a  load  there  is  no 
difference  between  a  "short-circuit"  and  a  " slip-ring"  armature, 
since  the  windings  of  the  slip-ring  armature  are  short-circuited, 
finally.  Different  windings  of  a  3-phase  motor  armature  suitable 
for  use  with  slip-rings  are  shown  in  Figs.  294  and  297. 


FIG.  294. — Wound  Rotor  of  Three-phase  Motor  (Korting  Brothers). 


The  input  of  an  induction  motor  can  be  measured  by  watt-meters 
and  the  output  by  a  proney  brake.  Thus,  the  ratio  of  input  to  output 
is  easy  to  obtain.  These  values  can  be  obtained  with  the  motor 


296 


ELECTRICAL  ENGINEERING 


standing  still,  in  which  case  the  output  is  0,  since  there  is  no  motion, 
but  the  torque,  or  tendency  to  turn,  is  a  specific  value  which  can  be 
measured.  Thus,  curves  of  current  input,  torque  (or  tendency  to 
turn) ,  and  speed  can  be  plotted  all  the  way  from  rest  up  to  synchro- 
nism, as  well  as  efficiency  and  power  factor  (i.e.  cosine  of  angle  of  lag 
of  entering  current,  as  has  been  explained),  and  maximum  output 
when  running  at  normal  speed.  The  curve  of  the  torque  from  rest 
to  synchronism  is  shown  in  Fig.  295.  Curve  shown  with  0.04  ohm 


FIG.  295. 

resistance  in  armature  is  so  chosen  that  the  torque  at  starting  is  the 
maximum  torque.     The  formula  for  torque  of  an  induction  motor  is 


Torque  m  pounds 


when  E0  =  the  applied  E.M.F.; 

p  =  number  of  field  (an  armature)  circuits  (thus  2  for  quarter- 

phase  and  3  for  three-phase)  ; 
s  =  slip  of   secondary   (at   standstill,  s=l;   at    synchronism, 

8  =  0); 

R=  resistance  per  circuit  of  armature  in  ohms; 
n=  cycles  per  second  in  primary; 
HO  =  resistance  per  circuit  of  primary  in  ohms  ; 
LI  =  inductance  per  circuit  of  armature  in  ohms  ; 
LQ  =  inductance  per  circuit  of  field  in  ohms. 

From  this  formulae  the  resistance  for  a  given  type  can  be  calculated. 


MULTIPHASE  ALTERNATING  CURRENT  297 

Curve  with  0.005  ohm  gives  the  torque  values  without  resistance  in 
the  armature.  It  can  be  seen  at  starting  the  torque  is  now  less  than 
with  resistance.  A  little  farther  the  two  curves  cross  and  have  the 
same  torque.  At  this  point  the  resistance  should  be  cut  out  and 
the  torque  up  to  synchronism  would  then  be  on  the  curve  higher. 
Thus,  under  such  conditions  the  complete  torque  curve  would  be  as 
in  curve  with  0.04  ohm  resistance,  the  resistance  being  cut  out  at 
the  point  where  the  two  curves  cross.  From  an  inspection  of  the 
formula  for  torque  it  can  be  seen  that  the  applied  voltage  appears 
in  the  numerator  as  the  square.  Thus,  the  torque  of  an  induction 
motor  is  proportional  to  the  square  of  the  applied  voltage.  Hence, 
low  voltage  on  an  induction  motor  means  more  than  lower  starting 
(or  running)  torque.  The  formula  for  the  horse-power  of  an  induction 
motor  is 


746[(R1+SR0)2  +  S' 

In  this  formula  it  can  be  seen  that  again  the  applied  voltage  EO 
appears  as  the  square,  so  that  the  output  of  an  induction  motor  when 
running  on  half-voltage  is  only  one-quarter  of  the  value  when  running 
on  full  voltage.  The  formula  for  the  maximum  horse-power  obtain- 
able from  an  induction  motor  is 


1492[(Ri  +  Ro)  +     (Hi  +  R0)2 

when  Xi  =  2xriLi 
and      XQ  =  27inLQ. 

The  formula  for  maximum  torque  in  pounds  at  one  foot  is 

^ 


Maximum  torque  = 


34.09[R0  +  V  R02 


From  an  inspection  of  this  formula  it  can  be  seen  that  RI,  the 
resistance  of  the  secondary,  does  not  appear,  from  which  it  may  be 
concluded  that  the  maximum  torque  of  an  induction  motor  is  inde- 
pendent of  the  secondary  resistance,  the  presence  of  the  latter  only 
determining  at  what  per  cent,  of  synchronism  the  maximum  torque 
will  appear.*  Curves  of  amperes  and  torque  without  resistance  and 
with  armature  short-circuited  are  shown  in  Fig.  295;  as  can  be  seen 
when  the  amperes  are  0;  that  is,  running  at  exact  synchronism,  there 

*  See  Raymond's  "Alternating  Current  Engineering,"  pages  128-163,  •*'>!• 
proof  of  these  formulae  without  using  calculus. 


298 


ELECTRICAL  ENGINEERING 


xis  no  torque;  when  standing  still  (or  0  per  cent,  synchronism)  the 
current  is  a  maximum.  Thus,  an  induction  motor  standing  still 
without  resistance  in  the  armature  takes  its  maximum  current, 
usually  about  ten  or  fifteen  times  its  normal  current. 

The  curves  showing  the  efficiency,  maximum  output,  etc.,  are 
shown  in  Fig.  296. 

It  will  be  noticed  that  the  output  reaches  a  maximum  in  this 
case  at  the  line  of  about  195  per  cent,  output,  beyond  which  no  more 
load  can  be  put  on  the  motor.  If  an  attempt  is  made  to  do  so,  the 
motor  will  slow  down  and,  unless  the  load  is  relieved,  come  to  rest. 
Since  under  such  conditions,  that  is,  at  rest,  the  motor  takes,  as  has 


FIG.  296. 

been  shown,  many  times  normal  full-load  current,  it  will  soon  burn 
up  unless  the  current  be  taken  off.  Thus,  in  applying  loads  to  in- 
duction motors  they  must  be  chosen  of  such  values  that  they  are 
always  below  the  maximum  output  of  the  motor. 

The  power  factor  of  a  motor  is  the  cosine  of  the  angle  of  lag  of 
entering  current,  or  the  ratio  of  the  real  input  to  the  apparent  input, 
obtained  by  multiplying  volts  and  amperes  together,  not  allowing 
for  any  lag. 

The  commercial  efficiency  is  the  ratio  of  the  actual  energy  given 
out  to  the  actual  energy  taken  in. 

The  apparent  efficiency  is  the  ratio  of  the  actual  energy  output  to 
the  apparent  input,  obtained,  as  stated,  by  multiplying  volts  and  am- 
peres input  together,  not  allowing  for  their  phase  difference.  Thus,  it 
can  be  seen  that  the  power  factor  equals  the  actual  input  divided  by 
the  apparent  input.  A  properly  designed  induction  motor  of  about 
50  H. P.  should  give  from  ordinary  conditions  a  maximum  output 
of  100  H.P.,  a  power  factor  at  full  load  of  95,  and  efficiency  of  92.  In 
spite  of  this  limitation  of  maximum  output,  induction  motors  are 


MULTIPHASE  ALTERNATING  CURRENT 


299 


used  very  extensively  indeed  in  the  United  States  for  all  sorts 
of  power  purposes,  and  are  built  in  sizes  up  to  3000  horse-po\ver= 
They  are  used  in  the  same  application  exactly  as  our  direct- 
current  motors.  Thus,  you  will  find  them  on  hoists,  cranes,  street- 
cars, pumps,  driving  shafting  in  mills,  driving  tools,  the  design  of 
output  and  torque  being  such  as  to  properly  meet  the  conditions 
imposed.  The  advantage  of  the  use  of  an  induction  motor  over  that 
of  a  direct  current  is,  in  the  former  the  commutator  is  dispensed 
with.  Thus,  there  are  no  brushes  to  attend  to,  nor  any  of  the  com- 
mutator troubles  which  arise  with  direct-current  apparatus.  Also 
since  long-distance  transmission  is  always  alternating,  the  induction 
motor  can  be  used  by  stepping  down  from  the  line  voltage  to  a  safe 
operating  voltage,  whereas  with  direct-current  motors  the  A.C. 
long-distance  transmission  voltage  must  be  transformed  into  direct 
current  by  rotaries  or  motor  generator  sets,  with  increased  cost  of 
first  installation  and  maintenance  and  attendance  during  operation. 
These  facts  have  made  the  use  of  induction  motors  very  general. 


m 


FIG.  297. — Triphase  Rotor  (Korting  Brothers}. 

By  means  of  a  voltmeter  connected  to  two  brushes,  we  may 
observe  that,  before  closing  the  starting  switch,  there  is  a  considerable 
voltage  between  two  slip-rings,  which  nearly  disappears  after  short- 
circuiting  the  resistance.  According  to  the  number  of  wires  on  the 
rotor,  the  voltage  between  the  slip-rings  will  be  smaller  or  larger, 
and  will  be  nearly  equal  to  that  of  the  stator,  if  the  number  of  turns 
on  stator  and  rotor  are  alike. 

The  number  of  rotor  windings  will  of  course  be  selected  so  as  to 
avoid  a  dangerous  voltage  arising  between  the  slip-rings  whilst  starting 


300  ELECTRICAL  ENGINEERING 

the  motor.  Generally  the  limit  for  this  voltage  is  200  to  300  volts. 
But  even  this  voltage  might,  under  unfavourable  circumstances,  prove 
dangerous,  and  therefore  touching  the  slip-rings  whilst  starting  the 
motor  should  be  avoided.  After  the  motor  has  reached  its  full  speed, 
and  the  starter  has  been  short-circuited,  the  slip-rings  may  without 
hesitation  be  touched  with  both  hands,  since  the  voltage  existing 
between  the  slip-rings  of  a  short-circuited  rotor  is  very  small. 

Alternating-current  motors  only  (both  induction  and  synchronous 
types)  have  the  important  advantage,  that  the  feeding  current  is  led 
to  a  stationary  winding.  Hence  these  motors  can,  as  well  as 
generators,  be  built  for  high  tension.  Large  3-phase  motors  for 
2000  and  even  5000  volts  are  frequently  made.  For  these  high 
pressures  the  terminals  and  windings  of  the  primary  have,  of  course, 
to  be  well  protected  to  prevent  danger  to  life,  and  the  windings 
must  be  excellently  insulated. 


Slip 

If  between  a  slip-ring  and  a  starter  terminal  we  place  an 
ammeter,  we  are  then  able,  with  a  slip-ring  induction  motor,  to  make 
some  interesting  observations.  As  long  as  the  motor  runs  without 
load,  the  ammeter  indicates  a  very  small  current,  but  a  very 
evident  and  slow  oscillation  of  the  pointer  of  the  instrument  will  be 
noticed.  If  the  load  on  the  motor  is  increased,  the  deflection  of  the 
pointer  becomes  greater  and  its  oscillations  occur  more  quickly. 
The  number  of  the  swings  gives  us  a  direct  measure  for  the 
slip  of  the  armature.  If  l^he  armature  runs  synchronously  with 
the  field,  then  —  as  we  know  —  no  currents  at  all  can  be 
produced  in  the  armature.  On  the  other  hand,  if  the  armature 
of  a  4-pole  motor  (not  loaded)  remains  by  two  revolutions  per 
second  behind  the  field  speed,  then  this  has  the  same  effect  as  if  the 
4-pole  field  rotated  twice  round  the  stationary  armature.  Hence, 
in  the  armature  an  alternating  current  of  eight  alternations  per 
minute  is  produced.  The  ammeter  pointer  is  then  deflected  eight 
times  from  zero  up  to  a  maximum  value.  If  the  motor  is  fully 
loaded,  its  slip  then  being  40  revolutions,  we  can  observe  160 
oscillations  of  the  ammeter  pointer  per  minute.  Since  in  this  case 
the  oscillations  quickly  follow  each  other,  the  pointer  has  no  time  to 
return  to  its  zero  position. 

This  phenomenon  becomes  still  more  distinct  if,  instead  of  the 
usual  electro-magnetic  or  hot-wire  instrument,  we  employ  a  Deprez 
ammeter,  with  a  zero  in  the  middle  of  the  scale.  Then  we  see  the 
needle  deflected  from  zero — for  instance,  to  the  right,  then  to  the  left 
and  back  again  to  zero,  and  so  on.  With  a  4-pole  motor  a  double 


MULTIPHASE  ALTERNATING  CURRENT 


301 


movement  of  this  kind  of  the  pointer  corresponds  to  the  slip  of  one 
revolution. 

The  vector  diagram  of  an  induction  motor  is  similar  to  that  of 
a  transformer,  since  the  former  is  really  a  stationary  transformer 
with  a  movable  secondary,  bearings  being  provided  to  permit  revo- 
lution. It  may  be  that  a  vector  diagram  for  an  induction  motor 
cannot  be  drawn  as  in  the  case  of  a  transformer,  for  in  the  former 
case  the  secondary  has  the  same  frequency  as  the  primary,  while  in 
the  induction  motor  the  frequency  in  the  rotor  is  the  same  as  in  the 
primary  when  the  rotor  is  standing,  but  is  0  when  running  at  syn- 
chronism, and  about  4  per  cent,  of  full  frequency  when  running  at 
full  load.  It  must  be  borne  in  mind,  however,  that  the  revolutions  of 
the  rotor  plus  the  frequency  in  the  rotor  always  equal  exactly  the  pri- 
mary frequency.  From  the  nature  of  things,  thus  the  rotation  brings 
around  the  secondary  current,  so  that  when  they  are  at  a  maximum 
they  bear  physically  to  the  primary  the  same  relation  as  if  they  had 
full  frequency  and  the  rotor  were  standing  still.  In  vector  diagrams 
and  calculations,  it  is  convenient  to  reduce  the  induction  of  sec- 
ondary to  the  same  number  of  turns  as  the  primary  by  multiplying 
the  value  of  L  and  R  by  the  square  of  the  number  of  turns,  reducing 
similarly  back  again  by  dividing  after  the  calculation  is  over.  Fig. 


FIG.  298. 

293  is  so  drawn  and   gives  the  phase  relations  between  the  various 
currents  and  the  flux  in  an  induction  motor. 

Let  o-c  equal  in  length  and  phase  the  flux  (£.  Then  the  line  o-a 
equals  the  E.M.F.  produced  by  the  flux  due  to  its  pulsating  in  the 
primary,  and  o-b  the  E.M.F.  in  the  secondary  when  standing  still. 
Draw  o-k  ahead  in  phase  to  o-c  an  amount  such  that  the  product  of 
its  projection  upon  o-a,  the  E.M.F.,  gives  the  losses  in  the  motor  due 
to  friction,  hysteresis,  eddy  currents,  etc.  Thus,  the  product  of 
E.M.F.  and  current  in  phase  represents  energy.  Let  o-g  represent 
current  in  the  armature.  It  lags  behind  the  E.M.F.  o-b  by  the  angle 
b-o-g.  o-f  in  phase  with  the  current  represents  the  E.M.F.  used  up 
in  resistance.  (The  loss  in  resistance  is  always  in  phase  with  the 


302  ELECTRICAL  ENGINEERING 

current  from  Ohm's  law.)  o-e  drawn  90  degrees  away  from  the  current 
represents  the  E.M.F.  consumed  by  induction.  (Always  90  degrees 
away  from  the  current,  as  has  been  shown  earlier  in  the  book.)  Then 
o-d  represents  the  E.M.F.  at  this  load  necessary  to  force  the  current 
o-g  through  the  rotor,  since  resistance  drop  and  inductance  drop 
combine  by  the  parallelogram  of  forces,  as  has  been  shown.  The  line 
o-n  equals  the  secondary  current  as  it  appears  in  the  primary,  since 
the  currents  in  the  secondary  always  appear  equal  and  opposite  to 
the  primary  (assuming,  as  in  this  case,  a  ratio  of  terms  of  1:1). 
Thus,  the  total  primary  current  is  the  combination  of  the  exciting 
current  o-k  and  this  other  component  o-n,  since  there  are  no  other 
currents.  Thus,  o-p  equals  the  primary  current;  this  current  flowing 
through  the  primary  windings  consumes  the  E.M.F.  o-i  in  phase  with 
itself,  and  the  E.M.F.  o-h  at  right  angles  with  itself,  or  the  combina- 
tion of  both,  i.e.,  o-m.  The  primary  applied  E.M.F.,  therefore,  has  to 
be  of  such  a  value  and  phase  as  to  overcome  the  combination  of  the 
primary  impedance  drop  o-m  and  the  E.M.F.  produced  in  the  wind- 
ings by  the  flux  pulsating  through  them,  or  o-a.  The  combination 
by  the  parallelogram  of  force  of  o-a  and  o-m  equals  o-L,  which  rep- 
resents, therefore,  in  amplitude  and  phase  the  applied  E.M.F.  Thus, 
this  diagram  gives  the  value  and  phases  of  all  the  currents  and  E.M.F.'s 
existing  in  an  induction  motor. 


Single=phase  Induction  Motors 

After  the  invention  of  the  three-phase  motor  its  simplicity 
and  its  superiority  over  all  other  alternating-current  motors  soon 
became  known.  Since  then  many  alternating-current  central 
stations  have  been  designed  for  the  three-phase  system,  especially 
when  the  motor  load  is  important.  There  are,  however,  many  older 
central  stations  which  work  with  single-phase  current.  For  light- 
ing purposes  the  single-phase  system  has  an  advantage  over  the 
three-phase  system,  for  with  the  latter  it  is  rather  difficult  to 
distribute  the  lamps  so  as  to  get  equal  loads  on  the  three 
phases.  Further,  with  the  3-phase  system  three,  but  with  the 
single-phase  system  only  two,  mains  are  required.  Engineers  have, 
therefore,  given  much  attention  to  the  design  of  single-phase  induc- 
tion motors,  having  the  advantages  of  3-phase  motors. 

If,  whilst  a  2-phase  motor  is  running  lightly  loaded,  we  disconnect 
one  phase  from  the  motor,  we  observe  that  the  motor  still  continues 
to  run,  and  a  considerable  alteration  takes  place  in  the  current 
consumption,  both  in  the  connected  phase  and  the  armature,  but, 


MULTIPHASE  ALTERNATING  CURRENT 


303 


what  is  most  essential,  the  motor  continues  to  do  work.  From  this 
observation  we  conclude  that  it  is  feasible  to  build  single-phase 
induction  motors. 

On  stopping  the  motor,  and  trying  to  start  it  again,  with  only  one 
phase  connected,  the  armature  circuit  being  closed,  a  great  difference 
will  be  observed  between  this  and  an  ordinary  3-phase  motor.  A 
single-phase  motor  is  not  self -starting. 

We  can,  after  continued  experiments,  find  out  a  position  of  the 
lever  of  the  starting  resistance,  so  that,  when  the  rotor  is  once 
set  in  motion,  it  continues  to  run,  runs  quicker  and  quicker,  till 
finally,  if  we  gradually  short-circuit  the  resistance,  the  full  speed  is 
reached.  We  may  further  observe  that  we  can  give  the  motor  a 
start  either  to  the  right  or  to  the  left,  and  that  in  both  cases  it  will 
continue  to  run  in  the  direction  in  which  we  started  the  rotor, 
whereas  a  3-phase  motor  runs  when  the  connections  are  made  in 
a  definite  way  in  one  direction  only. 

With  the  single-phase  motor  we  have  originally  not  a  rotating, 
but  merely  a  pulsating  field.  Hence  there  is  no  turning  effort 

on  the  stationary  arma- 
ture, but  a  force  which 
tends  to  pull  the  rotor 
first  in  one  and  then  in 
the  opposite  direction. 
Similar  actions  take 
place  in  many  other 
machines.  A  steam-en- 
engine  furnishes  a  good 
example :  the  piston 
has  a  reciprocating  mo- 
tion, and  piston-rod, 
crank,  and  flywheel  are 
required  to  transform 
this  kind  of  motor  into 
a  rotating  one.  The 
bicycle  and  sewing- 
machine  are  similar  instances.  If  the  crank  (see  Fig.  299)  is  at  the 
top  or  bottom,  then  the  piston  is  unable  to  produce  any  motion,  neither 
by  pulling  nor  by  pushing.  It  is  absolutely  necessary  that  the  crank 
be  turned  past  these  dead  points.  If  the  crank  reaches  the  position 
shown  in  Fig.  300,  on  pushing  the  piston  a  further  turning  of  the  crank 
in  the  direction  indicated  by  the  arrow  will  result.  If,  on  the  other 
hand,  we  had  turned  the  flywheel  counter-clockwise  (see  Fig.  301) 
instead  of  clockwise,  then  a  thrust  on  the  piston  will  cause  a 
motion  of  the  crank  to  the  left.  Hence,  if  no  other  mechanism 
prevents  this,  we  can  really  turn  as  we  please  the  flywheel  of  a 
sewing-machine,  either  to  the  right  or  to  the  left.  When 'the  machine 


FIG.  299. 


FIG.  300. 


FIG.  301. 


304 


ELECTRICAL  ENGINEERING 


'Choking 
Coil 


Is  once  started,  then  we  are  assisted  by  the  momentum  of  the 
flywheel,  which  carries  the  crank  over  the  dead  points,  thus  giving  us 
the  desired  rotatory  motion. 

In  a  corresponding  way  the  working  of  a  single-phase  motor  may 
be  imagined.  We  have  first  of  all  to  help  the  armature  over  the 
dead  points,  in  order  to  produce  in  the  windings  of  the  armature 
(which,  as  with  the  3-phase  motor,  might  be  wound  as  a  squirrel- 
cage,  2-  or  3-phase),  by  its  revolution  in  a  pulsating  field,  currents 
of  different  phases,  which  latter  then  produce  a  rotating  field  in 
the  armature.  The  armature  wires  then  act  like  a  flywheel,  taking 
up  the  reciprocating  forces  and  producing  rotating  power. 

Small  motors  may  be  started  by  hand,  but  this  would  be  im- 
possible with  large  motors.  Such  motors  may  be  started  by  providing 
them  with  a  second  phase 
winding,  obtaining  by  its 
help  a  self-starting  single- 
phase  motor.  Through 
this  second  phase,  during 
starting,  a  current  is  caused 
to  flow  which  differs  in 
phase  from  the  current  in 
the  main  phase.  We  may 
get  a  second  phase  from 
a  single-phase  circuit  by 
dividing  the  main  current 
into  two  parts  and  insert- 
ing in  one  branch,  called 
the  '  auxiliary  phase,"  a 
choking  coil.  These  con- 
nections are  shown  diagrammatically  in  Fig.  302.  The  main 
phase  is  connected  to  the  mains  directly,  the  auxiliary  phase  is  in 
series  with  a  choking  coil.  In  the  latter  circuit,  due  to  the  large 
self-induction  of  the  choking  coil,  a  far  greater  phase-difference 
between  current  and  voltage  is  produced  than  in  the  main  phase. 
If  the  phase-difference  becomes  a  quarter-period,  then  we  get  a 
complete  rotating  field.  The  phase-difference  will,  however,  here  be 
far  smaller  than  a  quarter-period,  since  in  the  main  phase  the  current 
already  lags  behind  the  voltage,  owing  to  the  main  phase  not 
being  free  from  self-induction.  In  the  auxiliary  phase  the  phase- 
difference  is,  of  course,  larger,  but  in  no  case  as  much  as  a 
quarter-period.  Hence  we  do  not  get  a  true  rotating  field.  The 
effect  may  be  compared  to  the  swinging  pendulum  to  the  bob  of 
which  we  gave  a  lateral  push  before  it  reached  its  highest  posi- 
tion. Then  the  pendulum  will  not  get  a  circular,  but  an  elliptical 
motion. 

Hence  the  single-phase  motor  with  an  auxiliary  phase  is  self- 


FIG.  302.  — Single-phase  Motor  with  Auxiliary 
Phase  with  Self-induction. 


MULTIPHASE  ALTERNATING  CURRENT  305 

starting,  but  the  starting  power  so  obtained  is  far  smaller  than  that  of 
a  3-phase  motor.  Whilst  the  latter  can  start  under  full  load  and  even 
double  the  normal  load,  an  ordinary  single-phase  motor  with  auxiliary 
phase  can  only  start  with  a  part — say  about  one-third  to  two-third's 
of  its  normal  load.  By  suitable  means  the  starting  power  may  be 
increased,  but  then  the  current  consumption  is  greatly  increased. 

Single-phase  induction  motors  are  therefore  generally  provided 
with  a  loose  pulley.  Before  starting  the  motor  the  belt  is  placed  on 
the  loose  pulley,  so  that,  on  starting,  the  motor  has  to  overcome  the 
low  frictional  resistance  of  the  loose  pulley  only.  After  the  motor 
has  reached  its  full  speed  the  auxiliary  phase  is  switched  out,  and  the 
belt  is  removed  from  the  loose  on  to  the  belt-pulley.  Sometimes, 
instead  of  a  loose  pulley,  a  friction  coupling  is  employed.  When 
a  coupling  of  this  kind  is  fixed  the  motor  starts  without  any  load,  and 
after  it  has  reached  full  speed  the  coupling  is  thrown  into  gear,  either 
by  hand  or  automatically. 


Phase= Difference  caused  by  Capacity 

We  may  now  deal  with  another  kind  of  phase-difference  besides 
that  produced  by  self-induction. 

Any  cable  and  generally  any  conductor  \vhich  is  connected  with 
a  single  pole  of  a  source  of  alternating  current,  causes  a  so-called 
"charging-current"  to  flow.  Let,  in  Fig.  303, 
the  two  circles  I.  and  II.  represent  the  slip-rings 
of  an  alternating-current  generator,  the  brushes  of 
which  are  connected  with  wires.  The  two  wires 
are  not  connected  with  each  other.  As  long  as 
the  machine  is  stopped,  all  conductors  which  are 
considered  here — the  armature  conductors,  the 
brushes,  and  the  two  wires — have  the  same 
electrical  potential.  But  when  the  machine  is 
working,  in  each  of  the  two  brushes  an  alternating 
potential  or  pressure  appears,  sending  a  current 
to  the  end  of  the  wire,  which  is  still  at  the 
previous  potential.  If  we  had  a  continuous- 
current  dynamo,  then  this  current  would  soon 
FIG.  303.  cease — -the  positive  brush  sending  a  current 

through  the  wire  connected  with  it  until  the 
whole  main  had  the  potential  of  this  part;  similarly,  the  negative 
brush  would  take  current  from  the  main  connected  to  it  till  the 
lattter  came  to  the  same  low  potential  as  the  negative  brush  itself. 
When  this  state  is  reached,  the  charging  current  will  cease. 


306 


ELECTRICAL  ENGINEERING 


If,  on  the  other  hand,  we  have  a  perpetually  alternating  potential 
on  the  two  brushes  the  case  is  different.  As  long  as  the  voltage  on  the 
brush  increases,  it  will  send  a  current  to  the  end  of  the  wire.  The  wire 
is  now  charged,  and  will,  when  the  voltage  of  the  brush  after  reaching 
itfe  maximum  begins  to  fall,  return  to  the  brush,  like  an  honest 
debtor,  the  amount  previously  borrowed.  During  the  time  the 
voltage  of  the  brush  decreases  from  its  maximum  through  zero  to  its 
minimum  value,  the  current  comes  back  from  the  wire  to  the  brush; 
whilst  during  the  time  in  which  the  voltage  of  the  brush  increases, 
a  current  flows  from  the  brush  to  the  end  of  the  cable;  only  at  the 
instant  when  the  voltage  has  its  maximum  or  minimum  value 
is  the  current  zero.  Hence  the  charging  current  has,  exactly  like  the 
magnetizing  current,  a  phase-difference  of  a  quarter-period  from  the 
voltage.  If  the  voltage  has  its  maximum  value  the  charging  current 
is  zero,  and  if  the  charging  current  has  its  maximum  value  the 
voltage  is  zero.  The  charging  current  is  therefore  a  wattless  current, 
like  the  magnetizing  current.  It  is  a  quarter  of  a  period  in  advance 
of  the  voltage,  whereas  the  magnetizing  current 
is  a  quarter  of  a  period  behind  the  voltage. 

The  charging  current  will  be  greater  the  larger 
is  the  " capacity"  of  the  main  connected  with  the 
brush.     A  large  capacity  may  be  produced  either 
if  the  cables  are  very  long  or  if  they  lead  to  very 
large  surfaces  which  are  placed  directly  opposite 
to  each  other,  if  for  instance  (see  Fig.  304),  the 
wires  I.  and  II.  are  connected  to  very  large  sheets 
of  tinfoil,  which  are  fixed  on  opposite  sides  of  a 
glass  plate  or  a  sheet  of  mica.  By  this  means  the 
capacity   of  the  mains  is  increased   considerably. 
Such  an  apparatus  is  called  a  condenser.     Instead 
of   using   a   single  large   sheet   of   glass   or   other 
material,  a  number  of  smaller  plates  connected  in 
parallel  may  be  employed  with  the  same  effect. 
A  type  of  condenser  in  ordinary  use  consists  of  a 
large  number  of  sheets  of  tinfoil,  insulated  from 
each  other  by  mica  or  sheets  of  paraffined  paper;     FIG.  304.— Capao- 
the  alternate  sheets  of  foil,  say  the  1st,  3rd,  5th,        [iY in  Circuit, 
etc.,   are   connected   to   one   terminal,   whilst   the 
2nd,  4th,  6th,  etc.,  go  to  a  second  terminal.     When  a  condenser 
is  put  across  the  mains  supplied  with  alternating  current,  a  current 
flows  in  and  out  of  it,  although  the  two  halves  of  the  condenser  are 
insulated  from  each  other.     To  get  large  currents  from  the  mains 
in  this  way  necessitates  the  use  of  a  very  large  number  of  sheets 
of  foil. 

The  effect  of  capacity  may  very  well  be  observed  with  long,  and 
more  especially  with  concentric,  mains.     Such  cables  cause    a  con- 


MULTIPHASE  ALTERNATING  CURRENT 


307 


Condenser 


(Capacity) 


FIG.  305. — Single-phase  Motor  with  Auxiliary 
Phase  having  Capacity. 


siderable  current  to  flow  from  the  alternator  to  the  mains,  even  if 
not  a  single  lamp  or  apparatus  is  switched  on.  This  current  is, 
however,  as  already  mentioned  wattless. 

This  capacity  effect  may  be  used  instead  of  a  choking  coil  for 
producing  a  phase-difference  in  the  auxiliary  phase  of  a  single-phase 

induction  motor  (see 
diagram,  Fig.  305) . 
Theoretically  with  this 
arrangement  it  would 
be  quite  possible  to  pro- 
duce a  perfect  rotating 
field;  for  the  main  phase 
has  a  certain  self-induc- 
tion, and  therefore  a 
phase  retardation  of  the 
current  occurs  in  it, 
whereas  in  the  auxiliary 
phase  in  which  a  con- 
denser is  inserted,  a  lead 
and  not  a  lag  of  the 
current  takes  place. 
With  a  condenser  of  the 

right  capacity  a  phase-difference  of  a  quarter-period  with  the  same 
current  could  be  effected,  and  thus  a  proper  rotating  field  produced. 
To  secure  that  sufficient  current  passes  to  the  condenser,  it  is 
necessary  to  have  much  capacity,  which  means  a  very  large  and 
expensive  arrangement.  With  the  usual  construction  of  condensers 
for  these  purposes  a  combined  capacity  and  resistance  effect  is  used. 
The  condenser  plates  are  placed  in  a  tank  filled  with  a  conducting 
liquid,  such  as  a  solution  of  soda.  The  phase-difference  is  in 
this  case  far  less  than  a  quarter  of  a  period.  It  therefore  follows 
that  motors  provided  with  such  condensers  cannot  start  under  a 
heavy  load. 

Sometimes  both  induction  and  capacity  are  employed  for  starting 
a  single-phase  motor,  a  choking  coil  being  inserted  in  one,  a  condenser 
in  the  other  phase. 

A  phase-difference  of  the  currents  in  the  two  windings  may  also 
be  produced  by  inserting  in  one  winding  ohmic  resistance  only.  Then 
in  this  phase  the  phase-difference  between  current  and  voltage  will 
not  be  as  large  as  it  is  in  the  other  phase. 

Even  without  a  resistance,  choking  coil  or  condenser,  the  starting 
of  a  single-phase  motor  may  be  effected  by  providing  the  main  and 
the  auxiliary  phase  with  a  very  different  number  of  windings  and 
switching  them  directly  on  the  mains.  Since  the  self-induction 
in  the  two  windings  will  then  be  different,  the  two  currents  will 
also  have  different  phase-differences  against  the  outer  voltage. 


308  ELECTRICAL  ENGINEERING 

All  the  means  that  have  been  described  in  which  capacity  or  self- 
induction  effects  help  to  create  a  rotating  field,  are  used  only  for 
starting  single-phase  motors.  After  this  has  been  effected  these 
devices  are  switched  out  of  the  circuit.  The  motor  is  then  really 
running  on  a  single  phase  only,  but  by  the  effect  of  the  armature 
conductors  a  rotating  field  is  then  produced. 

How  to  build  single-phase  motors  with  considerable  rotary  power, 
and  which  can  be  overloaded  without  stopping,  like  3-phase  motors,  is 
a  problem  that  has  yet  to  be  solved  in  a  satisfactory  way.  If  it  were 
possible  to  design  single-phase  motors  with  the  same  good  working 
properties  as  3-phase  motors,  then  the  3-phase  system  would  probably 
be  soon  discarded. 


The  Reversing  of  Alternating=current  Motors 


For  reversing  single-phase  motors  without  an  auxiliary  phase  no 
change  of  the  connections  is  required.  It  has  no  inherent  tendency 
to  rotate  in  a  definite  direction,  the  direction  in  which  it  is  turned 
by  hand  or  by  any  auxiliary  means  decides  the  matter.  This  applies 
both  to  single-phase  synchronous  motors  and  to  single-phase  induction 
motors  without  an  auxiliary  phase. 

The  single-phase  induction  motor  with  auxiliary  phase  behaves 
exactly  like  a  2-phase  motor,  and  we  shall  therefore  first  of  all 
examine  the  behaviour  of  the  latter.  In  considering  the  working  of 
a  2-phase  motor  we  have  assumed  (see  Figs.  278  and  279)  that 
phase  A  produces  at  one  moment  a  north  pole  on  the  left;  phase  B,  a 
quarter  of  a  period  afterwards,  a  north  pole  below.  Hence  the  field 
rotates  from  the  left  downwards,  then  to  the  right,  then  upwards,  i.e. 
in  counter-clockwise  fashion.  We  may  now  reverse  the  direction  of 
rotation  of  the  field  in  different  ways :  firstly  by  reversing  the  current 
in  phase  A.  The  effect  of  this  will  be  that  phase  A  will  not,  at  the 
particular  moment  considered  above,  produce  a  north  pole  on  the  left 
but  on  the  right.  Phase  B,  however,  the  connections  of  which  have 
not  been  altered,  produces  now,  as  before,  a  quarter-period  later, 
the  north  pole  at  the  bottom.  Hence  we  have  now  a  rotation 
from  the  right  downwards,  to  the  left,  then  upwards,  i.e.  a  clockwise 
rotation. 

We.  may,  of  course,  get  the  same  effect  by  leaving  unaltered  the 
current  direction  in  A,  and  reversing  that  in  B.  Reversing  the  current 
both  in  A  and  B  would,  of  course,  be  ineffective. 

The  third  way  to  reverse  the  motor  is  to  interchange  the  phases, 
so  that  then  phase  B  is  traversed  by  that  current,  which  has  a  lead 


MULTIPHASE  ALTERNATING   CURRENT 


309 


of  one-fourth  of  a  period.  Then  B  will  at  first  produce  a  north  pole 
below,  and  a  quarter-period  later  A  will  produce  a  north  pole  to  the 

left,  so  that  we  now  get  a 
clockwise  rotation,  i.e.  we  have 
reversed  the  previous  direction 
of  rotation. 

We  may  hence  alter  the  di- 
rection of  rotation  of  a  2-phase 
motor  having  four  terminals, 
simply  by  changing  the  two 
mains  of  one  phase.  With  a 
2-phase  motor  supplied  with 
three  mains  (the  middle  one 
having  about  1£  times  the  area 
of  the  outer  mains)  it  is  only 
necessary  to  change  the  posi- 
tions of  the  outer  mains. 

When  we  wish  to  reverse 
the  direction  of  rotation  of  a 

single-phase  motor  with  an  auxiliary  phase,  either  the  two  ends  of  the 
main  phase  or  those  of  the  auxiliary  phase  must  be  changed.  In  Fig. 
306  the  diagram  of  connections  for  clockwise  rotation  is  given.  One 
main  is  connected  with  end  I/  of  the  main  phase  and  with  end  II.  of 


FIG.  306. — Connection  of  Single-phase 
Motor  for  clockwise  Rotation. 


FIG.  307.  FIG.  308. 

Connections  of  Single-phase  Motor  for  Counter-clockwise  Rotation. 

the  auxiliary  phase.  The  connection  for  counter-clockwise  rotation 
may  then  be  made  either  according  to  Fig.  307  or  to  Fig.  308.  In 
the  former  case  the  direct  main  is  connected  with  I.  and  II.,  whereby 
the  ends  of  the  main  phase  are  changed.  In  the  latter  case  the  direct 
main  is  connected  with  I/  and  II.',  causing  the  ends  of  the  auxiliary 
phase  to  be  changed. 


310  ELECTRICAL  ENGINEERING 

With  a  3-phase  motor  the  reversal  of  rotation  may  be  effected 
by  changing  the  ends  of  any  two  of  the  three  mains,  it  being  a 
matter  of  indifference  whether  the  motor  has  either  a  star  or  a  mesh 
connection.  Let  us  consider  the  motor  to  be  star-connected,  and  the 
rotating  field  to  be  produced  as  shown  in  Figs.  287,  289,  290,  and 
291.  Let  us  further  assume  that  the  currents  in  the  phases  B  and 
C  are  interchanged;  then  we  arrive  at  the  following  result:  Previously, 
the  strongest  north  pole  was  produced  first  on  the  top,  then  below  to 
the  left,  next  below  to  the  right,  etc.,  giving  a  field  rotation  in  a 
counter-clockwise  direction.  When  now  the  change  is  made,  the 
north  pole  is  produced  firstly  at  the  top,  then  below  on  the  right,  next 
below  on  the  left,  and  so  on;  hence  the  field  is  rotating  clockwise. 
The  same  result  may  be  effected  by  changing  the  phases  A  and  B  or 
C  and  A. 

These  alterations  of  connections  refer  of  course  only  to  that  part 
of  the  motor  fed  by  the  alternating  current.  Altering  the  con- 
nections in  the  rotor  of  an  induction  motor  or  in  the  magnet 
system  of  a  synchronous  motor  are  without  effect  on  the  direction  of 
rotation,  because  the  armature  of  an  induction  motor  or  the  magnet 
system  of  a  synchronous  motor  are  always  made  to  revolve  with  the 
rotating  field  that  is  produced  by  the  supplied  alternating  current. 

In  the  United  States  slip-rings  are  used  only  in  special  cases. 
Instead,  the  resistance  is  mounted  within  the  armature  itself,  thus 


FIG.  309. 

revolving  with  it,  and  no  slip-rings  are  required,  the  connection 
between  windings  and  resistance  being  direct.  The  resistance  is  cut 
out  by  sliding  contact  brushes,  which  are  pushed  forward  and  back 
on  contacts  mounted  on  the  resistance  (sometimes  the  contacts  rest 
upon  the  wire  of  the  resistance  itself) ,  by  means  of  a  collar  mounted 
upon  the  shaft.  This  collar  has  in  it  a  groove  in  which  the  lugs 
from  the  starting  lever  are  located.  Thus,  the  collar  which  revolves 


MULTIPHASE  ALTERNATING  CURRENT  311 

with  the  shaft  only  makes  contact  with  the  lugs  while  they  are  pushing 
the  collar  forward  or  back.  The  motion  imparted  to  the  collar  is 
thus  transmitted  to  the  brushes  and  the  resistance  altered  while  the 
motor  comes  up  to  speed,  being  finally  cut  out  at  full  speed.  This 
arrangement  avoids  all  troubles  from  collector-rings.  The  latter  are 
used  when  the  resistance  needs  to  stay  hi  circuit  all  the  time  and 
when,  therefore,  the  resistance  must  be  so  large  that  there  is  not 
enough  room  within  the  armature  to  place  it.  Fig.  309  shows  an 
induction-motor  armature  with  internal  resistance  revolving  with 
the  armature  itself,  thus  avoiding  collector-rings. 


Faults  with  Alternating-current  Motors 

If  we  try  to  reverse  the  direction  of  a  3-phase  motor  by  changing 
the  two  ends  of  one  phase  as  we  did  with  the  2-phase  and  the  single- 
phase  motor  with  auxiliary  phase,  the  result  will  be  of  interest.  The 
motor  is  then  no  longer  a  3-phase  motor,  and  it  either  does  not  run 
at  all,  or  only  with  a  third  of  its  normal  speed,  consuming  at  the 
same  time  an  excessive  current,  and  soon  getting  very  hot.  For  in 
this  case  phase  A  will,  at  the  moment  of  its  maximum  strength, 
produce  a  north  pole  at  the  bottom;  a  third  period  later  phase 
B,  which  is  still  connected  as  before,  will  produce  a  north  pole  below 
to  the  left,  and  again,  after  a  third  period,  phase  C  will  produce  a 
north  pole  below  to  the  right. 

With  a  slip-ring  motor  we  may,  by  observing  the  armature 
voltage,  perceive  exactly  whether  the  motor  has  a  correct  rota  ting- 
field  connection  or  not.  With  a  properly  connected  2-  or  3-phase 
motor  a  lamp  connected  with  the  armature  slip-rings  will  burn 
regularly  as  long  as  the  armature  circuit  is  not  closed  (see  p. 
295).  The  position  of  the  armature  does  not  make  any  difference, 
since  the  field  rotates  with  a  uniform  speed  about  the  stationary 
armature.  With  a  single-phase  motor,  however,  we  have  no  rotating 
but  merely  a  pulsating  field,  and  thus,  according  to  the  position  of 
the  respective  armature  coil  in  the  pulsating  field,  the  lamp  will  burn 
either  brightly  or  with  little  light  or  will  not  burn  at  all.  The  same 
will  occur  with  any  irregular  rotating  field;  thus,  for  instance,  with 
a  single-phase  motor  with  auxiliary  phase,  or  with  the  erroneous 
connection  of  a  3-phase  motor  just  mentioned.  Hence,  if  we  con- 
nect a  lamp  or  a  voltmeter  with  two  slip-rings  of  the  armature,  and 
with  a  slow  rotation  of  the  armature  we  observe  that  the  voltage 
between  the  two  slip-sings  varies  considerably,  then  we  infer  that 
there  is  something  wrong  with  the  3-phase  motor.  If  one  of  the 


312  ELECTRICAL  ENGINEERING 

3  phases  is  disconnected,  so  that  the  motor  is  on  2  phases,  then 
the  same  phenomenon  may  be  observed  as  with  the  single-phase 
motor. 

There  is  another  fault  sometimes  found  in  the  working  of 
induction  motors.  If  one  phase  of  the  armature  is  disconnected — 
for  instance,  if  one  of  the  brushes  is  not  in  contact  with  its  slip-ring— 
then  the  motor  may  run  at  half  speed.  If  this  happens  we  can  easily 
make  sure  whether  there  is  a  disconnection  in  the  armature  itself,  in 
the  brushes  or  in  the  starter  circuit,  by  examining,  with  a  lamp  or  a 
voltmeter,  firstly  the  voltage  between  each  two  of  the  three  slip- 
rings,  then  the  three  brush-holders,  etc.  If  the  disconnection  is 
within  the  armature,  then,  if  the  brushes  are  taken  off  and  the 
stator  windings  are  switched  in,  no  voltage  can  be  observed  between 
one  of  the  slip-rings  and  either  of  the  two  others;  hence  a  lamp 
connected  with  this  slip-ring  does  not  bum,  and  a  voltmeter  is  not 
deflected. 

In  the  case  of  an  induction  motor  without  slip-rings,  the  fact 
that  one  of  the  phases  of  a  three-phase  motor  is  connected  in  reversal 
by  mistake  can  be  noted  by  the  fact  that  the  motor  will  not  come 
up  to  full  speed.  Then  the  three  currents  entering  the  motor  are 
not  alike,  as  they  should  be,  but  one  smaller  and  the  other  two  con- 
siderably larger,  and  often  at  starting  a  considerable  humming  will 
be  noticed.  Another  fault  with  induction  motors  is  a  sudden  shut- 
down and  resulting  blowing  of  fuses.  Investigation  may  show  all 
circuits  to  be  O.K.  In  fact,  the  motor  may  have  been  running  satisfac- 
torily for  some  time.  Often  the  cause  will  be  found  to  be  due  to  the  rui>- 
bing  of  the  field  on  the  armature.  Air-gaps  of  induction  motors  must 
be  as  small  as  possible  in  order  to  get  good  power  factors  for  the 
magnetizing  current,  which,  as  has  been  shown,  lags  behind  the 
applied  E.M.F.  and  thus  lowers  the  power  factor.  The  magnetizing 
current  is  used  principally  in  an  induction  motor  in  forcing  the  lines 
of  force  through  the  air-gap,  the  iron  parts  of  the  circuit  not  counting 
much.  Thus,  the  smaller  the  gap  the  better.  In  motors  as  large 
as  500  H.P.  the  gap  is  only  .050  inch,  and  in  small  motors,  such  as 
10  H.P.,  this  may  be  only  .015  inch.  Thus,  in  the  case  of  shut- 
downs and  blowing  of  fuses,  the  air-gap  should  be  investigated.  If 
rubbing  is  occurring  the  H.P.  consumed  by  the  rubbing  may  be 
such  that,  added  to  the  regular  H.P.,  the  total  may  be  beyond  the 
maximum  output  of  the  motor  itself,  thus  causing  the  shut-down. 
This  touching  when  it  first  occurs  is  not  noticeable,  since  it  is  slight. 
As  it  increases  a  point  may  be  reached  where  actual  trouble  results. 
This  rubbing  is  also  most  injurious  to  the  windings ;  since  the  energy 
represented  by  it  is  shown  as  load,  it  may  destroy  the  insulation, 
introducing  short-circuits,  still  further  complicating  matters.  In  a 
plant  using  induction  motors  an  examination  of  the  air-gaps  once  a 
month  does  not  consume  much  time  and  guards  against  trouble. 
Low  voltage  on  the  line  is  another  cause  of  induction-motor  trouble, 


MULTIPHASE  ALTERNATING   CURRENT 


313 


since,  as  has  been  shown,  the  output  of  an  induction  motor  is  pro- 
portional to  the  square  of  the  voltage.  If  the  latter  is  low,  a  large 
effect  on  the  output  results.  Hence,  if  a  motor  has  swings  of  load 
carrying  it  up  for  a  moment  to  something  near  its  maximum  out- 
put, it  may  break  down  under  such  load  conditions,  if  the  voltage 
be  low.  The  same  holds  true  in  starting  a  motor;  if  the  voltage  is 
low,  a  low  starting  torque  results.  Another  cause  of  low  voltage 


FIG.    310.  —Forty-pole  Armature  of  Tri-phaser  (Korting  Brothers}. 

of  a  motor  is  unbalanced  voltages,  while  a  motor  may  give  proper 
output  with  balanced  voltages.  If  these  become  much  unbalanced, 
the  output  is  much  reduced. 

Finally,  short-circuits  in  the  fields  are  a  source  of  considerable 
bother  with  poor  insulation.  Such  a  short-circuited  coil  does,  not 
burn  out  at  once,  since  by  Lenz's  law  the  current  induced  in  it  by  the 
pulsating  flux  opposes  the  flux.  About  three  times  normal  E.M.F. 
flews,  but  does  not  burn  up  the  coil  at  once,  but  creates  a  local  heating, 


314  ELECTRICAL  ENGINEERING 

which  may  affect  other  coils  until  finally  the  motor  becomes  in- 
operative. In  a  plant  using  motors  it  is  well  to  measure  the  insu- 
lation resistance  of  all  motors  (and  lines)  once  or  twice  a  month  to 
locate  such  faults  soon  after  they  appear. 

Transmission  of  Multiphase  Currents 

The  two  currents  produced  in  a  2-phase  generator  may  be 
separately  led  through  two  pairs  of  wires  to  a  2-phase  motor.  In 
this  case  four  mains  are  required,  each  of  which  has  to  carry  the 
single  current.  This  is  shown  schematically  in  Fig.  311.  The  zig- 


FIG.  311. — Two-phase  System  with  four  Mains. 

zag  lines  represent  the  phase  windings  of  the  generator  and  motor 
respectively.  In  order  to  indicate  the  2-phase  system,  the  zigzag 
lines  are  at  right  angles  to  each  other. 

It  is,  however,  possible  to  combine  two  of  these  mains.  We  can 
join  the  ends  I.  and  II.  of  the  generator,  and  1  and  2  of  the 
motor  (see  Fig.  312).  Then  each  phase  has  one  main  for  itself,  but 
the  middle  main  is  common  to  both  phases,  and  through  this 
main  the  currents  flowing  in  the  two  outer  mains,  return  together. 
Now  one  might  think  that  the  current  flowing  in  the  middle  main 
must  be  twice  as  great  as  that  flowing  in  one  of  the  outer  mains. 
This  is  not  the  case,  since  the  two  currents  are  different  in  phase,  and 
therefore  do  not  arrive  at  their  maxima  simultaneously.  This  may  be 
seen  from  Fig.  313.  On  adding  the  values  of  the  two  wave-lines,  by 
first  plotting  the  heights  of  one  wave  and  above  them  the  heights  of 
the  second  wave,  we  get  a  resultant  wave,  which  is  marked  in  the 
figure  by  a  thick  line.  We  observe  that  this  resultant  wave  is,  of 
course,  of  greater  amplitude  than  either  of  the  single  waves,  but  only 
1^  times  or,  as  may  be  found  by  an  exact  calculation,  1.41  times  as 
great.  Not  only  is  the  maximum  value  of  the  combined  current 


MULTIPHASE   ALTERNATING  CURRENT 


315 


1.41  times  as  much  as  the  maximum  value  of  the  single  currents,  but 
this  is  also  true  regarding  the  effective  value  of  the  combined  current; 
that  is  to  say,  it  is  1.41  times  as  large  as  the  effective  value  of  the 


H1  2' 

FIG.   312. — Two-phase  System  with  three  Mains. 

single  currents.     For  this  reason  the  sectional  area  of  the  middle  wire 
must  be  made  equal  to  about  1£  times  the  area  of  the  single  wires. 

We  have  now  to  consider  what  will  be  the  voltage  between  these 
two  interlinked  phases — that  is,  between  the  terminals   I'  and  II". 


FIG.  313.  —  Resultant  of  two  Alternating  Currents  differing  in  Phase 
by  one-quarter  of  a  period. 


As  will  readily  be  understood,  this  voltage  will  not  be  twice  as  great 
as  the  separate  voltages,  but  also  about  1.41  times  as  much. 

This  will  be  clearer  from  the  following  comparison.  Suppose 
a  man  walks  from  a  (Fig.  314)  100  yards  in  a  straight  direction, 
reaching  a  point  o.  At  o  he  makes  a  quarter  of  a  turn,  and 
then  goes  100  yards  up  to  6.  Although  now  the  man  has  gone 


316 


ELECTRICAL   ENGINEERING 


2  x  100  =  200  yards,  he  is  not  at  a  distance  of  200  yards 
from  the  starting-point,  but,  as  can  be  found  by  measuring  exactly 
the  connecting-line  ab,  a  dis- 
tance of  141  yards  only. 

This  geometrical  figure  can 
be  used  in  another  way.  We 
can  measure  on  the  line  ab  the 
resulting  voltage  between  the 
outer  terminals  (the  interlinked 
voltage  of  the  system)  provided 
that  we  make  the  length  of  the 
other  two  sides  of  the  triangle 
correspond  to  the  phase  voltages. 
If,  for  instance,  for  100  volts 
phase  voltage  we  make  the  sides 
oa  and  ob  equal  to  100  inches, 
then  the  length  of  the  line  ab 
will  be  equal  to  141  inches— 
which  means  that  the  interlinked 
voltage  of  this  2-phase  system 
is  equal  to  141. 

With  3-phase  machines  we 
may  lead  six  mains  from  the 
generator  to  the  motor,  as 
shown  in  Fig.  315.  The  phases 
of  both  machines  are  indicated 
by  zigzag  lines  at  angles  of  120°. 


100 
FIG.  314. 

Now  we  may,  as  we  have  done  before,  combine  the  returns.  Thus 
we  have  to  connect  the  inner  ends  of  the  three  phases  of  motor 
and  generator  with  each  other,  getting  consequently  three  single 
leads  and  one  common  return  (see  Fig.  316). 

Next  let  us  consider  what  will  be  the  voltage  between  the  outer 
terminals  and  the  current  in  the  common  return.  The  first  of  these 
two  questions  may  easily  be  answered  by  means  of  a  drawing  similar 
to  that  shown  in  Fig.  314.  Let  us  plot  three  straight  lines,  distant 
from  each  other  by  120°  (see  Fig.  318).  The  line  oa  represents 


MULTIPHASE  ALTERNATING  CURRENT 


317 


the  voltage  of  the  first  phase,  ob  the  voltage  of  the  second,  and  oc 
that  of  the  third  phase.  To  get  the  voltage  between  the  outer 
terminals  of  the  nrst  and  second  phase,  we  have  only  to  connect  a 


and  b  by  a  straight  line,  and  to  measure  the  length  of  the  latter. 
If  the  lines  oa  and  ob  have  a  length  of  100  inches,  then  the 
dotted  lines  ah,  cb,  and  ca,  will  have  a  length  of  173  inches  each 


318 


ELECTRICAL  ENGINEERING 


/       ^ 


173 

FIG.  318. 


Thus  between  the  outer  terminals  of  a  3-phase  generator  so  con- 
nected a  voltage  of  173  will  appear  when  the  phase  voltage  is 
100.  The  interlinked  voltage  is  therefore  equal  to  1.73  times  the 
phase  voltage. 

With  regard  to  the  current  flowing  in  the  common  return, 
we  arrive  at  a  curious  result.  In  Fig.  288,  the  three  single  currents 
of  a  3-phase  system  have  been  represented  by  three  wave-lines, 

a,  b,  and  c  respectively.     To  get 
n  the  resultant  current,  we   have, 

as  in  the  case  of  Fig.  280,  to  add 
the  three  currents.  Hence  we 
have  to  plot  on  each  vertical  line, 
starting  from  the  horizontal,  first 
the  height  of  wave  a  at  this 
point ;  and  shall  then — according 
as  the  waves  6  and  c  are  directed 
at  this  point,  upwards  or  down- 
wards— plot  their  heights  above 
or  below  the  zero  line  respectively. 
In  doing  so,  we  are  surprised  to 
find  that  we  always  arrive  at  the 
horizontal  middle  line.  If,  for 
instance,  a  reaches  its  maximum 

upwards,  then  the  waves  b  and  c  are  directed  downwards,  each 
having  half  the  height  of  the  wave  directed  upwards.  It  is  now 
obvious  that  if  a  person  ascends  a  height  of  100  yards,  and  then 
descends  twice  50  yards,  he  will  come  back  to  the  level  from  which 
he  started;  also  that  at  any  moment  the  waves  b  or  c  have  their 
upper-  or  lower-most  position,  and  in  any  point  between  these 
positions  the  same  will  occur.  Thus  through  the  middle  wire  no 
current  flows  at  all. 

This  seems  very  strange  indeed,  and  one  might  ask  what  has 
happened  with  the  three  currents  which  are  flowing  tin  ~>ugh  the 
three  outer  mains.  The  answer  is  very  simple.  If  the  current 
passing  out  of  the  phase  OI  of  the  generator  (see  Fig.  316)  is  just 
at  a  maximum — say,  for  instance,  100  amps.,  and  is  directed  outwards, 
then  the  currents  in  the  two  other  phases  have,  as  we  have  seen 
from  the  wave-line  in  Fig.  288,  an  opposite  direction  and  only  half 
the  strength  or  50  amps.  Hence  from  I.  to  1  a  current  of  100  amps, 
is  flowing,  which  passes  through  the  phase  1 — 0  of  the  motor,  then 
branches  in  two  parts,  so  that  50  amps,  are  flowing  through  each  of 
the  two  other  phases,  passing  then  through  the  two  mains  2 — II.  and 
3 — III.,  coming  back  again  along  the  generator  phases  II. — 0  and  III. 
— 0,  to  the  common  centre  point  of  the  generator.  Sinre  now4:he  current 
which  passes  outwards  through  one  main  comes  back  again  through 
the  two  other  mains,  the  return  O — 0  is  useless;  it  is  a  neutral  main, 


MULTIPHASE  ALTERNATING   CURRENT 


319 


and  can  be  left  out  altogether.  In  Fig.  317  a  3-phase  system 
with  three  mains  is  shown,  as  is  usually  employed  if  the  generator 
is  used  for  feeding  motors  only.  Between  any  two  of  the  three 
mains  the  voltage  is  of  the  same  value. 

The  neutral  or  middle  wire  is  often  employed  in  cases  when 
both  motors  and  lamps  are  installed  on  the  3-phase  mains  (see 
Fig.  319).  The  motors  are  then  directly  fed  by  the  outer  mains, 
which  might,  for  instance,  have  a  voltage  of  173;  the  lamps  are 


FIG.   319. — Glow  Lamps  star-connected. 

connected  between  one  outer  and  the  neutral  wire,  which  have  a 
voltage  of  100. 

Very  frequently  the  phase  voltage  is  in  such  a  system  110,  the 
interlinked  voltage  being  then  110  X  1.73  =  190. 

If  one  or  several  lamps  are  connected  across  a  single  phase  only 
(see  Fig.  320),  then  obviously  through  this  phase  more  current 


0 


FIG.  320. — Glow  Lamps  on  one  Phase. 

flows  than  through  the  two  others.  The  balance  is  then  disturbed, 
and  the  neutral  wire  has  to  carry  some  current.  If  now  this  one 
phase  were  loaded,  and  the  two  other  phases  were  not  loaded  at  all, 


320  ELECTRICAL  ENGINEERING 

then  the  neutral  wire  would  have  to  carry  the  full  current  of  the  first 
phase.  This  is  also  the  case  with  currents,  the  waves  of  which  are 
irregular,  when,  even  if  the  mains  are  equally  loaded,  current  may 
flow  through  the  neutral  wire. 

The  arrangement  of  the  three  phases  of  a  3-phase  system  so 
far  described  is  called  the  star  method.  It  is  essential  with  star 
connections  that  the  beginnings  of  the  3-phase  windings  are  con- 
nected together  at  one  point.  From  this  point,  the  neutral  point, 
the  three  phases  radiate  like  the  rays  of  a  star. 

The  phases  may  also  be  arranged  so  that  in  turn  the  end  of  the 
first  is  connected  with  the  beginning  of  the  second  phase,  the  end  of 
the  second  with  the  beginning  of  the  third,  and  the  end  of  the  third 
with  the  beginning  of  the  first  phase,  as  shown  in  Fig.  321.  We 
get  in  this  way  the  mesh  or  delta  connections.  Considering  Fig.  321, 


FIG.  321. — Mesh  Connection  of  Machine   and  Load. 

one  would  expect  that  through  these  closed  windings  a  strong 
current  must  flow,  even  if  there  is  no  outer  load  at  all.  This  is  not 
the  case.  As  previously,  with  the  star  connections,  the  three  currents 
added  together  destroyed  each  other,  so  the  three  voltages  adaed 
together  give  no  voltage.  The  matter  is  somewhat  more  complicated, 
but  similar  to  the  phenomenon,  which  we  have  studied  in  the  case  of 
the  Gramme  armature  (see  p.  76),  where  we  have  a  closed  circuit 
in  which  electromotive  forces  are  acting.  These  E.M.F.'s  are  equal, 
but  opposed  to  each  other,  so  that  their  resultant  is  zero. 

Any  side  of  the  triangle  (Fig.  321),  and  therefore  any  phase,  has 
its  voltage,  and  we  may  take  current  from  the  machine  by  connecting 
between  any  two  mains  a  conductor,  for  instance,  a  lamp.  If  the 
lamps  are  equally  distributed  between  the  mains  I.  and  II.,  II.  and 
III.,  III.  and  I.,  then  through  all  the  mains  equal  currents  flow. 
The  current  passing  through  any  one  of  the  mains  will  then  of  course 
be  equal  to  the  sum  of  the  currents  flowing  in  the  two  phases,  which 


MULTIPHASE   ALTERNATING  CURRENT  321 

are  connected  with  this  main.  Let  us  now  suppose  that  the  phase 
currents  are  100  amps.  each.  It  might  at  first  be  thought  that  the 
main  will  receive  200  amps.  But  it  must  be  remembered  that 
between  the  currents  in  the  single  phases  there  is  a  considerable 
lag,  causing  their  resultant  to  be  only  173  amps. 

Hence  with  the  mesh  connection  the  voltage  between  two  outer 
mains  is  equal  to  the  phase  voltage,  but  the  current  in  the  outer 
mains  is  1.73  times  as  great  as  the  phase  current. 

With  the  star  connection  the  voltage  between  the  outer  mains 
(the  resultant  voltage)  is  1.73  times  the  phase  voltage,  but  the 
current  in  the  outer  mains  is  equal  to  the  phase  current. 

Let  us  compare  the  amount  of  copper  required  to  transmit  a 
certain  amount  of  energy  over  a  three-phase  circuit  with  the  amount 
of  copper  for  the  same  percentage  loss  using  a  single-phase  circuit, 
the  condition  of  comparison  being  that  the  voltage  between  lines  be 
the  same  in  each  case.  In  the  single-phase  circuit  let  the  percentage 
line  drop  equal  S,  and  the  power  transmitted  equal  P.  Let  L  equal 
length  of  line  both  ways,  E  equal  volts  between  lines,  and  I  the  current 

QTT 

flowing  into  the  line.    Then  the  resistance  of  the  line  equals  ^y-,  and 

QT? 

the  resistance  per  foot  equals  -yy. 

LI 

In  the  three-phase  circuit  the  energy  in  watts  per  circuit  equals 
p 
—  .     We  will  imagine  for  a  moment  that  there  are  three  separate 

o 

-p 

circuits,  each  having  a  voltage  equal  to  -=,  that  is,  the  voltage  between 


any  outside  wire  and  the  neutral.    The  current  per  circuit   (thus 

p       "p1         p 
divided  into  three  separate  circuits)  equals  Ii  equals  —  +  -—  = 

Thus,  Ii  equals  —/=,  when  I  is  the  single-phase  current  for  power  P. 

For  the  same  percentage  drop,  S,   we    have    a  voltage  drop  of 

SE  .  SE        I       SE 

-=  or  a  resistance  drop  of  —^=+—==  —  ,  and  the  resistance  per  foot 
V3  \/3     V3       I 

f  SE     L     2SE      L   .  ,  . 

of  —  —  -±-—  =—  —  .    —  is  used  instead  of  L,  since,  as  has  been  shown, 

1         Z        Ll          L 

on  a  three-phase  circuit  the  return  wires  become  unnecessary  and 
can  be  left  out  of  the  calculation.     With  the  single-phase  circuit  the 

QTT 
resistance  per  foot  was  shown  to  be  yp    Thus,  with  the  three-phase 

circuit  one  of  the  wires  has  twice  the  resistance  of  one  of  the  wires 
in  a  single-phase  circuit.     Hence,  since  the  resistance  of  w  wire  is 


322  ELECTRICAL  ENGINEERING 

usually  proportional  to  its  area  or  weight,  the  weight  of  the  wire  in 
the  three-phase  circuit  is  one-half  that  of  the  single-phase  circuit. 
But  there  are  three  wires  in  the  three-phase  and  only  two  in  the 
single-phase.  Thus,  if  the  weight  of  a  single  wire  equals  W  in  the 
single  phase,  the  total  weight  single-phase  equals  2W,  since  there 

3W 
are  two  wires,  and  the  total  weight  three-phase  equals  — .     Hence, 

3W 

the  ratio  of  weight  three-phase  to  weight  single-phase  equals  —  -T-  2W 

=  |.  Thus,  for  a  given  difference  of  potential  between  wires,  it 
takes  only  three-fourths  the  weight  of  copper  at  a  given  percentage- 
line  loss  to  carry  the  energy  three-phase  as  compared  with  carrying 
it  single-phase.  On  this  account  it  is  customary  in  long-distance 
transmission  to  carry  the  energy  on  three-phase  transmission  lines. 
They  have  a  further  advantage  that  the  self-induction  with  un- 
balanced loads,  while  unbalancing  the  various  voltages  between  the 
various  lines  somewhat,  does  not  do  so  to  the  extent  of  a  quarter- 
phase  transmission  with  a  common  return  wire.  For,  in  the  latter 
case,  as  can  be  shown  by  plotting  the  vector  diagram  and  remembering 
that  the  induction  is  at  right  angles  to  the  current,  it  is  seen  that 
the  inductance  of  the  common  wire  boosts  the  voltage  of  one  phase 
and  lowers  the  other. 


Power  in  a  Three-phase  System 

We  are  now  in  a  position  to  determine  the  output  of  a  3-phase 
generator,  if  the  voltage  and  current  be  given.  Assuming  the  phase 
voltage  to  be  100,  the  phase  current  10  amps.,  and  assuming  further 
that  the  load  consists  of  glow  lamps,  and  is  therefore  inductionless  ,  the 
calculation  becomes  very  simple.  Each  phase  supplies  100  volts  X 
10  amps.  =  1000  watts.  Hence  the  generator  supplies  3X1000  = 
3000  watts. 

The  calculation  becomes  apparently  more  complicated  if  there  be 
given  not  the  phase  voltage  and  the  phase  current,  but  either  (with 
star  connection)  the  resultant  voltage  and  the  phase  current,  or  (with 
mesh  connection)  the  phase  voltage  and  the  resultant  current.  In 
this  case  the  calculation  is  not  really  difficult.  Let  the  resultant 
voltage  be  173  and  the  phase  current  10  amps.,  then  to  get  the  phase 
voltage  we  have  to  divide  the  resultant  voltage  by  1.73.  Next  we 
have  to  multiply  the  phase  voltage  by  the  phase  current,  thence 
getting  the  output  of  one  phase.  This  we  have  again  to  multiply 
by  3  in  order  to  find  the  output  of  the  generator. 


Thus  |4|X10X3=100X10X3  =  3000  watts. 


MULTIPHASE   ALTERNATING  CURRENT  323 

[It  may  be  remarked  here,  that  the  phrases  "resultant  voltage"  and  "phase 
current"  are  very  seldom  used.  In  speaking  of  the  voltage  of  a  star-connected 
generator  the  resultant  voltage,  and  in  speaking  of  the  current  the  phase  current, 
is  generally  understood.] 

We  might  just  as  well  have  proceeded  in  another  way,  and  firstly 
have  multiplied  the  current  by  the  voltage,  next  by  3,  and  then 
have  divided  the  whole  by  1.73.  On  dividing  3  by  1.73  we  find 
that  we  get  1.73.  Hence,  instead  of  first  using  the  multiplier  3, 
and  afterwards  the  divisor  1.73,  we  can  directly  employ  the  factor 
1.73.  Our  rule  is  then  simply  to  obtain  the  product  of  volts, 
amperes,  and  1.73,  getting  thus  the  output  of  the  generator  in  watts 
when  the  load  is  free  from  self-induction. 

EXAMPLE. — 173  voltsXlO  amps.  X  1.73=3000  watts.  (Exactly  2992.9,  but 
3000  is  quite  accurate  enough.) 

The  same  calculation  applies  to  a  mesh-connected  generator. 
Let  the  voltage  be  again  100  and  the  resultant  current  1.73  amps. 
Then  we  get  the  phase  current  by  dividing  the  resultant  current  by 
1.73.  Next  we  have  to  multiply  the  phase  current  by  the  phase 
voltage  and  again  this  product  by  3.  Instead  of  dividing  by  1.73  and 
Multiplying  by  3,  it  is  easier  to  use  the  factor  1.73  as  before,  getting 
then  the  same  result  as  above.  Hence  we  may  say: — With  an  in- 
ductionless  load  a  3-phase  generator  has  an  output  in  watts  given  by 
the  formula — 

1.73XEXC 

where  E  is  the  voltage  and  C  the  current  of  the  system.  The  formula 
only  applies  if  the  three  phases  are  equally  loaded,  otherwise  it  is 
necessary  to  determine  the  output  of  each  phase  separately. 

If  the  load  of  the  generator  is  not  free  from  self-induction — if, 
for  example,  the  generator  has  to  feed  asynchronous  motors — then 
we  get  by  the  above  formula  not  the  real,  but  the  apparent  watts,  and 
we  have  still  to  multiply  the  result  by  the  power  factor  cos  (f>,  and 
the  formula  becomes — 


Watts  =  1 . 73  X  voltage  X  current  X  power  factor 
=  1.73  EC  cos  </>. 

A  3-phase  motor  which  is  supplied  at  190  volts,  taking  a  current 
of  12  amps.,  and  having  a  power  factor  of  0.8,  consumes  apparently — 

1.73 X 190 X  12  =  3944  (volt-amps.) 
and  really — 

3944 X  0.8  =  3155  watts. 


324 


ELECTRICAL  ENGINEERING 


Synchronizer  for  Multiphase  Machines 

The  connections  of  synchronizing  lamps  for  3-phase  current  are 
similar  to  those  for  single-phase  current.  For  low  voltages  three 
lamps  may  be  connected 

between      the      terminals  A  'A' 

AA',  BB',  and  CO'  (see 
Fig.  322).  If  all  three 
lamps  become  simulta- 
neously dark  or  bright, 
then  the  connections  are 
all  right,  and  at  an  instant 
of  darkness  the  switch 
may  be  closed.  It  might, 
however,  happen  that,  on 
starting  the  machines, 
or  after  any  alteration  on 
the  machine  or  switch- 
board, the  lamps  do  not 


FIG.  322. — Arrangement  of  Synchronizing 
Lamps  for  Three-phase  Circuit. 


become  bright  or  dark  simultaneously,  but  one  after  the  other.  This 
is  a  sign  that  the  succession  of  the  cables  on  the  terminals  of  the 
one  machine  does  not  correspond  with  the  succession  of  cables  on 
the  terminals  of  the  other  machine.  In  this  case,  any  two  of  the 
cables  may  be  changed;  for  instance,  that  cable  which  was  pre- 
viously connected  with  the  switch  terminal  A'  might  now  be  con- 
nected with  B',  and  that  of  B'  with  A'. 

In  still  another  way  we  can  assure  ourselves  before  starting  that 
the  cables  of  the  two  machines  are  correspondingly  connected  with 
the  switch,  viz.,  by  switching  a  3-phase  motor  first  on  the  terminals 
A,  B,  and  C,  and  then  in  exactly  the  same  way  on  A'B'C'  (that  ter- 
minal of  the  motor  which  was  on  A  before  now  to  be  connected  with 
A',  B  before  now  with  B').  If  the  direction  of  rotation  is  the  same 
in  the  second  case  as  it  was  in  the  first  one,  then  the  connections  are 
all  right;  if  the  direction  of  rotation  is  opposite,  then  two  of  these 
cables  must  be  changed  as  before. 

About  the  proper  connection  of  the  two  machines  we  have  to 
make  sure — once  for  all — before  switching  them  in  parallel  the  first 
time.  In  subsequent  work  it  is  quite  sufficient  that  one  phase  of  one 
machine  is  synchronous  with  the  corresponding  phase  of  the  second 
machine,  for  in  this  case  it  is  certain  that  also  the  two  other  phases  of 
the  first  machine  are  in  synchronism  with  those  of  the  second  one. 
Hence,  for  the  normal  working  of  the  machine,  three  synchronizing 


MULTIPHASE  ALTERNATING  CURRENT 


325 


lamps  are  not  required  as  is  shown  in  Fig.  3'J2,  but  two  or  one  respec- 
tively, as  with  a  single-phase  system. 

For  high  tension  likewise,  only  a  single-phase  transformer  is  re- 


*W\A< 


FIG.  323  — Arrangement  of  Synchronizing  Lamps  for  High-tension  Three-phase 

Circuits. 

quired,  which  then  is  switched  between  two  mains.     This  arrange- 
ment, shown  in  Fig.  323.  corresponds  to  that  of  Fig.  266. 

The  connections  of  a  2-phase  synchronizer  are  arranged  in  exactly 
the  same  way. 


CHAPTER  XII 
HIGH  TENSION 

WITH  alternating-current  work,  very  frequently  high-tension  current 
has  to  be  considered,  and  we  shall  therefore  now  deal  with  the  safety 
appliances  and  arrangements  which  have  to  be  provided  both  for 
protecting  human  life  and  machines  and  apparatus  against  the 
dangerous  effects  of  high-voltage  currents. 

The  windings  of  first-class  machines  are  always  insulated  with 
the  very  best  insulating  materials.  Notwithstanding  this,  windings 
may  be  spoiled  if  metal  or  carbon  dust  or  damp  is  allowed  to  remain 
on  them.  Hence  the  first  condition  is  to  keep  the  machines 
always  clean.  To  keep  them  dry  is,  however,  not  always  possible. 
Especially  during  erection  or  long  disuse,  dampness  of  the 
windings  in  rooms  that  are  not  very  dry  can  hardly  be  prevented. 
Before  a  machine  is  started,  it  may  therefore  be  necessary  to 
dry  it  thoroughly.  This  refers  both  to  continuous-current  and 
alternating-current  machines. 

With  alternating  current  generators  drying  is  very  easily  effected. 
For  this  purpose  the  machine  has  to  be  short-circuited,  i.e.  all  the 
terminals  are  directly  and  without  any  outer  circuit  connected  ^\ith 
each  other,  and  an  ammeter  is  inserted  in  either  all  or  only  in.  cne 
phase.  Afterwards  the  machine  is  started,  and  the  magnet  field 
feebly  excited,  so  that  the  current  produced  by  this  weak  field  is 
equal  or  somewhat  larger  than  the  normal  current.  The  windings 
get  warmed,  and  if  the  machine  be  run  for  several  hours,  perfect 
drying  of  the  windings  can  be  effected. 

The  voltage  obtained  with  a  short-circuited  alternating-current 
generator  is  negligibly  small  whenever  the  ends  of  each  phase  are 
connected  directly  with  each  other.  This  is,  for  instance,  the  case 
with  a  short-circuited  single-phase  generator,  a  2-phase,  a  mesh-con- 
nected 3-phase  generator,  and  also  with  a  star-connected  generator, 
the  star-point  of  which  is  short-circuited  together  with  the  three 
outer  terminals.  In  all  these  cases  there  is  practically  no  voltage 
in  the  short-circuited  machines.  With  a  star-connected  3-phase 
machine,  on  the  other  hand,  in  which  the  outer  terminals  only  are 

326 


HIGH  TENSION  327 

connected  with  each  other,  there  might  under  certain  circumstances 
be  considerable  voltages.  With  such  high-tension  machines,  even 
when  short-circuited,  one  must  avoid  touching  the  windings. 

Synchronous  motors  may  be  dried  in  the  same  way  as  generators, 
by  short-circuiting  the  alternating-current  terminals,  driving  the 
machine  as  a  generator,  and  exciting  the  field. 

Induction  motors  may  be  heated  by  reducing  the  generator 
voltage  to  a  small  value,  short-circuiting  the  starting  resistances 
of  the  motor,  and  putting  a  brake  on  the  armature.  Then,  despite 
the  low  voltage,  a  considerable  current  will  flow  both  through  the 
stator  and  the  braked  rotor. 

In  a  corresponding  manner  static  transformers  may  be  treated 
by  short-circuiting  the  secondary  winding  and  connecting  the  primary 
(high-tension)  winding  with  a  voltage  far  lower  than  the  normal 
pressure  (about  3  to  5  per  cent,  of  the  latter).  The  current  flowing 
through  the  coils  is  then  about  as  much  as,  or  a  little  greater  than, 
that  for  which  they  have  been  designed. 

Machines  and  transformers  may  easily  be  manufactured  so  that 
with  proper  management  they  remain  in  a  proper  state  for  a  very 
long  time.  But  with  ammeters  and  voltmeters  and  other  instru- 
ments it  is  very  difficult  to  provide  an  insulation  which  can  stand 
voltages  of  several  thousand  volts  with  certainty.  When  ammeters 
are  employed  in  high-tensicn  plants,  they  must  always  be  mounted 
on  a  very  good  insulating  base.  Voltmeters,  measuring  the  voltage 
directly  between  two  high-tension  terminals,  are  seldom  employed. 
Generally  measuring  transformers  are  used,  the  ratio  of  the  number 
of  high-  and  low-tensicn  windings  being  definitely  fixed  as  required. 
If,  for  instance,  the  primary  winding  has  100  times  as  many  turns 
as  the  seccndary,  then  at  a  primary  voltage  of  5000  on  the  terminals 
of  the  low-voltage  coil  a  voltage  of  50  will  appear.  Generally  the 
scale  of  the  reading  instrument  is  not  marked  with  the  secondary, 
but  with  the  primary  voltage,  so  that,  for  instance,  that  point  to 
which  the  pointer  of  the  instrument  is  deflected  with  50  volts  is 
marked  5000. 

There  are  also  measuring  transformers  for  ammeters,  the  "current 
transformers."  Hence  high-tension  switchboards  may  be  manufac- 
tured without  any  high-tensicn  apparatus  at  the  front.  Voltmeters, 
ammeters,  and  wattmeters  are  inserted  in  low-tension  circuits.  The 
measuring  transformers,  the  high-tension  fuses,  and  the  high-tension 
switches  are  placed  behind  the  board,  and  nothing  but  the  long 
insulated  handles  of  the  switches  project  at  the  front  of  the  board. 

In  connection  with  high-tension  switches  there  are  generally 
employed  special  devices,  so  that  on  opening  the  switch  there 
are  long  air  gaps  between  the  contacts,  hence  the  arc  that  is 
produced  on  breaking  a  high- voltage  current  is  destroyed  (see 
Fig.  324). 


328 


ELECTRICAL  ENGINEERING 


For  the  same  reason  high-tension  fuses  are  of  special  construction, 
an  d  frequently  of  considerable  length .    Gen- 
erally the  fuse  wires  are  enveloped  in  insu- 
lated safety  tubes,  by  which  splashing  of  the 
melted  fuse  wire  is  prevented  (see  Fig.  325). 

An  essential  feature  with  which  we 
have  still  to  deal,  is  the  safety  of  persons 
in  charge  of  high-tension  plants.  It  is  a 
matter  of  course  that  in  no  case  two 
terminals  of  different  voltages  must  be 
touched,  but  to  touch  even  a  single  high- 
tension  terminal  must  also  strictly  be 
avoided.  This  might  have  fatal  conse- 
quences for  a  man  standing  on  an  uninsu- 
lated place  if  there  existed  anywhere  an 
earth  connection  with  the  second  pole. 
With  alternating-current  machines  a  fatal 
shock  may  result,  although  the  whole  net- 
work may  be  very  well  insulated ;  for,  as  we 
have  learned  from  the  effect  of  capacity, 
there  is  even  in  a  wire  connected  to  a 
sinrle  pole  a  current  continuously  flowing 
in  and  out.  Hence,  if  a  person  in  contact 
with  earth  touches  one  pole,  he  will  receive 
a  current  flowing  to  earth,  and  even  this 
may  prove  fatal. 


FIG.  324.  —  High-tension 
Switch.  (The  Brush  Com- 
pany). 


FIG.    325.  — High-tension  Fuse  (The  Brush  Company). 


HIGH  TENSION  329 

Therefore,  if  it  is  necessary  to  touch  working  high-tension 
machines,  apparatus,  or  "live"  mains,  insulation  of  the  operator  is 
necessary — a  position  on  a  good  uninjured  india-rubber  plate  or  dry 
wood,  the  protection  of  the  hands  with  good  rubber  gloves,  and  the 
wearing  of  rubber  shoes  are  necessary  precautions.  With  high-tension 
plants  other  dangers  may  arise  in  addition  to  those  due  to  touching 
the  mains.  Assuming  that  a  high-tension  generator  or  transformer  is 
fixed  on  an  insulated  foundation,  and  that  one  pole  of  the  machine 
has  a  connection  with  the  frame.  If  the  insulation  of  the  windings 
is  uninjured,  then  obviously  there  can  be  no  short  circuit,  and  hence 
no  interruption  of  work  will  happen.  But  the  frame  of  the  machine 
is  now  in  connection  with  one  pole,  so  that  touching  the  frame  is, 
for  a  man  standing  on  the  earth,  just  as  dangerous  as  touching  one 
high-tension  terminal.  If  somewhere  in  the  network  there  is  another 
pole  earthed,  then  the  attendant  stands  in  a  manner  with  his  feet 
on  one  pole  of  the  high-tension  service,  touching  with  the  hands  the 
second  pole.  It  is,  therefore,  generally  specified  that  such  insulated 
machines  and  transformers  must  be  provided  with  an  insulating 
platform. 

There  is  still  another  means  of  protection  against  such  accidents — 
connect  all  machines  and  transformer  frames  with  earth.  This  might 
be  done,  for  instance,  by  fixing  a  copper  wire  to  a  machine  bolt,  and 
leading  it  to  an  earth  plate  or  to  the  water  pipe.  If  there  is  a  good 
connection  between  earth  and  the  machine  frame,  then  a  considerable 
potential  difference  cannot  appear  between  them,  and  the  frame 
may  be  touched  without  any  danger.  If,  now,  one  pole  of  the 
machine  at  any  time  touches  the  iron  frame,  and  the  other  pole  has 
anywhere  in  the  network  an  earth  connection,  then,  of  course,  an 
interruption  of  the  work  will  follow,  due  to  the  short  circuit,  but 
without  endangering  human  life.  It  is,  notwithstanding,  possible, 
with  a  bad  earth  connection,  for  a  considerable  voltage  to  exist 
between  the  iron  frame  and  the  earth,  and  therefore  the  very  best  con- 
nection between  earth  and  the  iron  part  of  the  machine  is  the  main 
requirement  with  high-tension  plants.  Where  a  good  earth  con- 
nection cannot  be  made,  the  machine  should  be  insulated  from 
the  earth,  but  in  this  case  an  insulated  platform  round  the  machine 
is  required. 

The  same  refers  also  to  switchboard  apparatus — either  an 
insulated  platform  in  front  of  the  switchboard  in  cases  in  which 
the  touching  of  all  parts  carrying  current  cannot  be  avoided,  or  earth 
connection  with  all  places  which  have  to  be  touched  must  be  made. 
In  the  latter  case  all  levers  of  the  high-tension  switches,  the  iron 
frame  of  the  board,  the  cores  of  the  measuring  transformers,  etc.,  must 
be  connected  with  earth.  If  there  are  high-tension  ammeters  on  the 
switchboard,  they  are  either  enclosed  in  an  insulating  box,  or  covered 


330  ELECTRICAL   ENGINEERING 

with  a  metal  box,  which  latter  is  then  connected  with  earth.    The 
metal  box  has,  of  course,  in  the  front,  a  glass  window. 

The  limit  at  which  the  voltage  becomes  dangerous  cannot  be 
stated  exactly.  Generally  voltages  up  to  500  volts  are  not  fatal, 
whilst  under  especially  unfavourable  circumstances  shocks  of  even 
200  volts  may  have  fatal  results. 


Lightning  Arresters 

With  any  overhead  electric  mains,  whether  they  carry  either  high 
or  low  tension,  continuous  or  alternating  currents,  it  is  necessary  to 
arrange  safety  appliances  to  guard  against  lightning  discharges. 
Lightning  consists  of  an  electric  arc  which  strikes  either  across  two 
clouds  or  a  cloud  and  any  object  on  the  earth.  Its  potential  is  always 
extremely  high,  and  sometimes  amounts  to  many  millions  of  volts.  It 
is  therefore  able  to  break  down  any  insulation,  and  to  cross  air  gaps 
in  a  way  that  would  be  impossible  in  the  case  of  relatively  low 
voltages.  Very  frequently  discharges  of  atmospheric  electricity  occur 
which  are  invisible  to  the  eyes  (so-called  dark  discharges),  but  which 
may  damage  electrical  machines  and  apparatus. 

From  many  observations  it  has  been  found  that  lightning  does  not 
consist  of  a  single  discharge,  but  that,  despite  the  short  time  of  dis- 
charge, a  frequent  alteration  of  the  current  direction  takes  place. 
Lightning  is  hence  really  an  alternating  current  with  an  extremely 
high  periodicity,  many  hundred  times  greater  than  that  of  the  usual 
alternating  currents.  Even  if  lightning  lasts  but  a  fraction  of  a 
second,  yet  it  changes  its  current  direction  in  this  short  time  many 
thousand  times. 

Now,  the  effect  of  self-induction  is  far  greater  with  a  high 
than  with  a  low  periodicity.  With  alternating  currents  of  usual 
periodicity  we  have  observed  strong  induction  effects  on  coils  with 
iron  cores,  whilst  with  currents  of  such  a  high  periodicity  as  with 
lightning,  even  on  coils  consisting  of  very  few  windings  and  having 
no  iron  cores,  we  can  observe  strong  self-induction  effects.  Since 
a  circuit  having  a  large  self-induction  produces  a  back  E.M.F.  which 
opposes  the  increase  of  the  current-strength,  this  is  equivalent  to 
adding  a  very  large  resistance  to  the  circuit.  Hence  if  we  provide 
for  the  lightning  discharge  two  paths,  one  of  them  leading  through  a 
coil,  and  the  other  one  through  an  air  gap  to  the  earth,  then  by  far 
the  greatest  part  of  the  discharge  will  flow  through  the  air  gap  to  the 
earth,  whereas,  on  account  of  the  high  inductive  resistance,  but  a 
small  part  will  flow  through  the  coil.  This  fact  has  been  used  to 
prevent  lightning  from  flowing  through  sensitive  apparatus,  and  for 
conducting  it  in  a  harmless  way  to  the  earth. 


HIGH  TENSION 


331 


FIG.  326— Lightning 
Arrester. 


The  simplest  type  of  lightning  arrester  is  shown  in  Fig.  326.  It 
consists  of  two  horn-shaped  thick  copper  wires 
placed  opposite  each  other  and  fixed  on  porce- 
lain insulators.  One  horn  is  connected  with 
the  supply  line,  from  the  other  one  ?  cable  is 
led  to  an  earth  plate.  The  two  horns  do  not 
touch  each  other.  The  least  distance  between 
them  varies,  according  to  their  position 
(whether  fixed  in  rooms  or  outside),  between 
-fa  to  \  inch.  The  usual  dynamo  voltage  can- 
not bridge  over  this  distance. 

Before  the  aerial  line  is  connected  to  any 
machine,  or  to  any  transformer,  a  coil  con- 
sisting of  several  windings  is  interposed.  This 
latter  does  not  offer  any  obstacle  to  continuous 
currents,  or  for  alternating  currents  of  the 
usual  periodicity.  If  a  discharge  of  atmospheric  electricity  into  the 
line  takes  place,  then  this  discharge  current  will  not  flow  through 
the  induction  coil,  which  offers  to  it  a  very  high  resistance,  but  will 

flash  over  the  air-gap  to  the 
second  horn,  where  it  finds  a 
direct  way  to  the  earth. 

Since  there  is  now  formed 
an  arc  between  the  two  horns, 
the  dynamo  current  could  also 
take  this  way.  But  this  bridg- 
ing of  the  horns  by  means  of 
the  arc  lasts  only  a  very  short 
time.  By  the  action  of  the 
stream  of  heated  air  the  arc 
rises  upwards,  becoming  longer 
(in  this  the  special  shape  of  the 
horns  assists),  till  finally,  like 
the  arc  of  an  arc  lamp,  the 
carbons  of  which  are  separated 
too  far  from  each  other,  it  is 
extinguished.  All  this  happens 
FIG.  327.—  Westinghouse  Lightning  during  a  brief  period,  and  after- 
wards the  plant  remains  with- 
out injury. 

A  type  of  arrester  for  alternating  current  in  use  at  the  present 
time  by  the  General  Electric  Company  consists  of  multi-air  gaps 
with  shunt  and  series  resistance.  The  air-gaps  are  carefully  spaced 
between  cylinders  of  non-arcing  metal,  and  part  of  the  gaps  shunted 
by  resistance.  The  whole  is  then  connected  in  series  with  resistance. 
This  type  of  arrester  for  2200  volts  is  illustrated  in  Fig.  328,  which 


332 


ELECTRICAL   ENGINEERING 


shows  both  the  multiple  and  series  connection.  In  Fig.  329  is  shown 
a  35,000-volt  three-phase  arrester  with  double-blade  disconnecting 
switches  for  Y-connected  neutral  grounded  circuits. 


FIG.  328. 


The  non-arcing  character  of  the  alloy  used  in  the  cylinders  reduces 
the  number  of  gaps  necessary,  and  aids  the  resistance  in  reducing  the 


FIG.  329 


HIGH   TENSION 


333 


destructive  effects  and  in  opening  the  resulting  arc.  Not  only  the 
high-potential,  high-frequency  discharges  of  lightning,  but  also  the 
smaller  charges  within  the  circuit,  are  discharged  across  the  gap. 
These  surges  within  the  system  are  caused  by  opening  or  closing  of 
feeder  switches,  switching  in  transformers,  and  sudden  variations  of 
load.  Records  of  discharge  are  made  by  inserting  a  small  square  of 
paper  between  two  adjacent  cylinders  in  each  line,  the  discharge 
puncturing  the  paper.  These  are  renewed  regularly.. 

In  theory,  the  cylinders  are  charged  electrostatically  until  the 
voltage  is  high  enough  to  break  down  the  air-gaps  in  succession, 
passing  the  charge  along  from  cylinder  to  cylinder,  thus  discharging 
th^  whole  system 


FIG.  330. — Lightning  Arrester  used  on  Railway  Apparatus. 

An  arrester  used  on  railway  apparatus  is  shown  in  Fig.  330.  The 
arrester  has  an  adjustable  spark-gap  between  two  electrodes  in  the 
field  of  an  electro-magnet.  One  electrode  is  connected,  through  the 
magnet  windings  and  a  small  non-inductive  resistance,  to  the  ground. 
The  other  electrode  is  connected  to  the  positive  side  of  the  circuit. 
Under  normal  conditions  no  current  passes  through  the  arrester  coil, 
but  any  arc  established  by  a  lightning  discharge  which  jumps  the 
gap  and  is  followed  by  current  from  the  generator  is  blown  out  by 
the  magnetism  induced  by  the  coil  of  the  blow-out  magnet. 

These  arresters  are  used  in  connection  with  kicking  or  choke  coils. 


334 


ELECTRICAL   ENGINEERING 


When  used  on  a  feeder  panel,  the  panel  is  equipped  with  a  kicking 
coil  made  of  bare  copper  rod  coiled  and  connected  between  the  main 
switch  and  the  circuit-breaker. 

It  is  desirable  to  isolate  the  arresters  from  the  switchboard. 

In  addition  to  those  just  described,  there  are  many  other  types 
of  lightning  arresters.  In  one  type  many  metal  and  mica  plates  are 
arranged  alternately  one  upon  the  other.  The  first  metal  plate  is 
connected  with  the  line,  the  last  one  with  the  earth.  This  row  is 
practically  an  insulator  for  a  low  voltage,  but  a  lightning  discharge 
glides  over  the  outer  surface  of  the  mica  and  metal  plates. 

Every  aerial  line  must  be  protected  by  a  lightning  arrester  before 
it  is  passed  within  any  building. 


Switchboards 

The  object  of  a  switchboard  is  to  concentrate  (or  concentrate 
the  means  of  controlling)  all  the  energy  developed  or  distributed  in 
a  station  for  the  purposes  of  control,  distribution,  measurement,  and 
protection.  It  is  best  located  so  as  to  give  the  operator  full  view 


FIG.  331. — A.  C.  Three-phase  Generator  and  Feeder  Panels. 


HIGH  TENSION  335 

of  the  machines  and  so  as  to  keep  the  cable  leads  between  the  board 
and  machines  as  short  as  possible.  Plenty  of  room  back  of  the  board 
should  also  be  provided  in  order  that  the  attendant  may  safely  in- 
spect, repair,  or  adjust  connections. 

It  is  usually  built  up  in  sections  or  panels  of  a  strong  insulating 
fire-proof  material,  upon  which  are  mounted  the  various  instruments 
and  devices.  Marble  and  slate  fulfil  these  requirements  and  are 
most  generally  used.  Slate,  however,  is  not  used  for  circuits  above 
1000  volts  unless  the  high-potential  carrying  parts  of  the  circuits  are 
insulated  from  the  panel.  This  is  due  to  the  fact  that  slate  is  strat- 
ified and  is  liable  to  have  veins  of  lower  insulating  qualities.  Slate 
pa  icls  are  finished  with  oil  or  black  enamel.  Natural  black  slate  oiled 
is  very  substantial,  is  easily  retouched  by  the  attendant  in  case  it 
becomes  marred,  and  harmonizes  with  the  finish  of  the  devices  mounted 
on  it.  Black  enameled  slate  gives  an  excellent  polished  surface,  but 
is  difficult  to  retouch  if  scratched  through  the  coating  of  enamel. 

Marble  is  stronger  and  a  much  better  insulator  than  slate.  Any 
kind  of  marble  with  a  polished  surface  will  show  oil  stains,  which 
renders  it  difficult  to  keep  it  looking  neat.  In  order  to  overcome 
this  trouble,  marble  boards  may  be  black  enameled  or  given  a  dull 
black  finish.  The  latter  finish  is  perhaps  the  more  durable  and  is 
also  more  easily  repaired. 

The  panels  are  supported  by  bolting  them  to  vertical  pieces  of 
l|-inch  gas-pipe  by  means  of  malleable  iron  clamps.  The  pipes  are 
held  upright  by  mounting  them  in  a  cast-iron  flange  at  the  bottom 
and  by  bolting  to  a  horizontal  brace  at  the  top.  This  pipe- work 
permits  of  many  adjustments  and  is  often  used  in  supporting  the 
devices  and  connections. 

In  mounting  the  instruments  and  devices  on  a  switchboard, 
nothing  should  be  used  which  has  no  other  use  than  ornamentation. 
Similar  instruments  and  devices  on  the  different  panels  are  located 
at  the  same  heights,  which  tends  to  give  the  board  a  symmetrical, 
uniform,  and  pleasing  appearance.  Circuit-breakers  and  fuses  are 
located  near  the  top  of  the  panel  in  order  that  any  arc  may  rise  with- 
out injury  to  the  adjacent  devices  or  to  the  attendant.  Just  beneath 
the  breaker  are  located  the  instruments  which  must  be  of  a  type 
not  easily  affected  by  a  stray  field.  About  the  middle  of  the  panel 
are  placed  the  rheostat  hand-wheel,  field  switches,  etc.,  with  oil 
switches,  large  recording  wattmeters,  and  relays  at  the  bottom, 
depending  upon  the  nature  of  the  panel.  In  stations  of  large  capacity 
it  is  convenient  to  use  electrically  operated  switches.  In  this  case 
a  controlling  board  is  used  in  the  shape  of  an  inclined  table,  with  the 
meters  and  instruments  located  on  vertical  panels  back  of  the  con- 
trolling board.  By  this  means  an  operator  can  stand  in  front  of 
the  controlling  board,  with  all  of  the  controlling  switches  within 
easy  reach  and  with  the  various  instruments  in  full  view. 


336 


ELECTRICAL   ENGINEERING 


The  illumination  of  the  board  is  best  when  it  can  be  provided 
for  from  that  of  the  station.  When  necessary,  however,  lamps  are 
mounted  at  the  top  of  the  panel,  but  this  is  open  to  the  objection 
that  it  does  not  give  uniform  illumination  and  reflects  in  the  attend- 
ant's eyes. 

The  switchboard  is  a  check  upon  the  efficiency  and  economy  of 
the  whole  station.  The  various  machines  were  designed  to  operate 
under  certain  loads,  and  the  board  must  be  laid  out  with  sufficient 
indicating  and  recording  instruments  to  determine  if  the  machines 
are  working  under  proper  load,  and  to  obtain  a  record  of  the  total 


FIG.  332. 


output.  Sufficient  protective  devices  must  be  provided  in  order  to 
protect  the  system  and  its  various  parts.  Where  these  devices  are 
automatic,  they  should  be  reliable  and  kept  in  good  order.  Other- 
wise they  are  liable  to  become  a  source  of  trouble,  resulting  in  shut- 
downs. 

Switchboards  are  usually  arranged  so  that  similar  panels  are 
together.  This  avoids  crossing;  of  leads,  with  the  liability  to  short- 
circuit,  fire  risk,  and  shut-downs.  Usually  the  generator  panels  are 
at  one  end  of  the  board  and  the  feeder  panels  at  the  other.  Fig.  331 
shows  a  typical  board  of  two  alternating-current  three-phase  gen- 


FIG.  333. 


337 


338  ELECTRICAL  ENGINEERING 

era  tor  panels  and  two  feeder  panels.  Switchboard  design  and  con- 
struction has  become  so  standardized  that  complete  boards  are  made 
up  from  standard  panels  to  meet  practically  all  conditions  of  gen- 
eration and  distribution  of  energy.  Boards  such  as  shown  in  the 
above  figure  may  be  used  on  potentials  up  to  and  including  13,200 
volts  and  may  be  of  either  slate  or  marble,  since  no  parts  carrying 
high  potential  are  mounted  on  the  panels. 

In  laying  out  a  station  it  is  necessary  to  lay  out  both  the  board 
and  the  cable  runs  carefully,  in  order  to  make  proper  provision  for 
the  protection  of  the  cables  and  the  location  of  the  board  with 
respect  to  the  machines.  Direct-current  cables  between  the  board 
and  the  machines  are  usually  supported  on  insulator  racks  under  the 
main  floor.  High-tension  conductors  from  the  alternator  should 
preferably  be  lead-covered,  varnished  cambric  or  paper  cable  laid  in 
ducts. 

Isolated  boards  are  made  up  for  use  in  small  plants  where  but 
one  or  two  machines  are  controlled.  In  this  case  the  feeder  switches, 
instruments,  circuit-breakers,  or  fuses  are  mounted  on  the  same 
panel  as  shown  in  Fig.  332.  This  shows  an  A.  C.  isolated  board  for 
two  machines  and  feeder  circuits.  Such  boards  are  also  largely 
used  for  isolated  motor  control. 

At  the  present  time  most  sub-stations  are  used  for  railway  work 
and  hence  use  railway  converters  in  connection  with  high-tension 
transforming  apparatus.  Such  stations  differ  only  in  the  number 
and  size  of  the  units  and  use  a  board  such  as  shown  in  Fig.  333. 
Here  the  breaker  is  shown  at  the  top  of  the  panel,  with  the  ammeter 
just  beneath.  In  the  middle  is  located  the  handle  for  field  rheostat, 
feeder  and  field  switches  just  beneath,  and  recording  wattmeter  at 
the  bottom.  This  does  not  differ  materially  from  the  standard 
railway  generator  panel,  while  the  feeder  panels  usually  omit  the 
rheostat,  field  switch,  and  recording  wattmeter. 

The  panels  are  connected  in  the  positive  side  of  the  circuit,  the 
rotary  converters  having  their  series  fields  connected  in  the  negative 
side.  No  negative  switches  are  required,  as  the  negative  side  of  the 
board  is  connected  directly  to  the  ground. 


INDEX 


Accumulator  apparatus,  191 

Accumulator  battery,  185 

Accumulator  cars,  196 

Accumulator  cells,  184;  end  cells,  186 

Accumulator  plates,  181,182;  negative, 
182;  positive,  182 

Accumulators,  179,  180;  Plante",  181; 
efficiency  of,  183;  applications  of, 
195;  cars  provided  with,  196 

Action,  electro -dynamic,  60 

Alternating-current  curve,  70,  234 

Alternating-current  generators,  247 

Alternating-current  motors,  270;  im- 
portant advantage  of,  300;  reversing 
of,  308;  faults  with,  311 

Alternating  currents,  71,  217;  angles 
concerned  with,  217;  experiments 
with,  221;  magnetic,  electro-dy- 
namic, and  chemical  effects  obtained 
with,  223;  current  strength  and 
voltage  of,  224;  induction  effects  of, 
225;  differing  in  phase,  315 

Alternation,  71 

Alternators,  247;  double-current,  248; 
inner-pole,  250,  251;  with  single 
lot  at  ing  magnet  bobbin,  252;  in- 
ductor type,  253,  261-264;  alternat- 
ing-pole type,  253  and  frontispiece; 
electro-motive  force  of,  253;  quarter- 

Ehase,  255;   three-phase,  255;   regu- 
ition  of,  256;  efficiency  cf,  257;  on 
a  non-inductive  load,  260;    switch- 
ing in  parallel,   265;    action  of  two 
in  parallel,  268;    withdrawing  one, 
269;    simplest  type  of,  270 
Ammeter,    electro-magnetic,    18,    19; 
hot-wire,   19,   20,   Deprez,   57,   133, 
300;     Weston,    58;     shunt   for,    59; 
with      central      zero     point,      194; 


measuring  transformers  for,  327; 
in  high-tension  plants,  327 

Ampere,  15 

Amperes,  number  of,  passing  through 
a  circuit,  23 

Ampere's  Rule,  11 

Ampere-turns,  11,  16 

Angles  concerned  with  alternating 
currents,  217 

Apparent  watts,  246 

Applications  of  accumulators,  195 

Arc,  207 

Arc  lamp,  207;  current  strength  of, 
208;  series,  209;  differential,  210; 
shunt,  210;  general  arrangement  of 
parts  of,  211;  the  Kfizik  or  Pilsen, 
211,  212;  enclosed,  214,  216;  more 
economical  than  glow  lamp,  214; 
magnetite  (General  Electric  Com- 
pany), 215,  216;  used  as  a  search- 
light, 215;  the  Bremer,  216;  the 
Cooper-Hewitt,  216 

Arc  lamp  resistance,  without  cover, 
213;  enclosed,  213 

Arc  lighter,  214 

Armature,  68;  Siemens,  69;  Gramme, 
74,  320;  closed-coil,  75;  toothed, 
83;  smooth,  83;  partially  wound, 
84;  finished,  84;  locomotive  motor, 
85;  of  multipolar  dynamo,  109; 
former- wound,  115,  117;  motor, 
133;  with  three  slots  per  pole,  249; 
four-pole,  with  single  slot  per  pole, 
250;  of  inductor  machine,  262; 
primary,  285;  slip-ring,  293;  squir- 
rel-cage, 293;  short-circuit,  295; 
induction-motor,  diagram  of,  310. 
See  also  Drum  armature  and  Rimj 
armature 

339 


340 


INDEX 


Armature  coil,  multiple-formed,  116; 
series-formed,  116 

Armature  reaction,  88;  with  motors, 
149 

Armature  resistance  loss,  131 

Armature  winding,  outer  and  inner 
wires  of,  75;  of  single-phase  alter- 
nator, 254;  of  three-phase  alter- 
nator, 258 

Astatic  instrument  for  switchboards, 
59 

Asynchronous  motors,  285 

Auer,  Dr.,  of  Vienna,  213 

Automatic  features  of  Type  M  Control, 
175 

Auxiliary  phase  of  single-phase  motor, 
304,  307 

Back  electro-motive  force,  134,  226 

Bar  magnet,  5 

Battery,  galvanic,  1,  2;  storage,  185; 
Edison,  186;  buffer,  195 

Battery  switch,  189,  190 

Bayonet  holder,  204 

Bicycle,  diagrams  of  working  of,  303 

Bipolar  dynamo,  71 

Blow-out,  magnetic,  177 

Boat,  moving,  action  of,  66 

Bobbin,  for  alternating-current  electro- 
magnet, 227;  exciting,  of  inductor 
machine,  262 

Boosters,  97,  189 

Brake,  electric,  175;   Proney,  295 

Branching  of  circuits,  29 

Break  spark,  222 

Breaks  in  a  circuit,  125 

Bremer  arc  lamp,  216 

Brushes,  69;  sparking  and  displace- 
ment of,  118;  carbon,  120;  copper, 
120 

Brush-holder,  carbon,  120;  copper 
gauze,  121;  with  metal  and  carbon 
brushes,  121 

Buffer  batteries,  195 

Buffer  effect,  276 

C  type  of  dynamo,  100 

Cables,  concentric,  43;   control,  174 

Calculation  of  resistance,  23 

Calibration,  18 

Calorie,  39 

Carbon-holder,     sliding     type,      120; 

swivel  type,  120 
Carbonizing,  204 
Carbon  lamp,  203 
Cast  steel,  103 
Cell,  galvanic,  1;  storage,  184 


Cell  switches,  186 

Cells,  in  series  and  parallel,  31;  in 
opposition,  32;  jointly  supplying 
an  outer  circuit,  32 

Cellulose,  203 

Centigrade,  24 

Centimetre  measure,  24 

Charging,  180,  194 

Charging  current,  305 

Chemical  energy,  197 

Choking  coil,  238,  304 

Circuit-breaker,  50-52 

Circuits,  magnetic,  15,  63,  86,  106,  163; 
simple,  27;  series,  29;  in  parallel, 
29;  branching  of,  29;  characteristic 
of  closed,  88;  main,  92;  shunt,  92; 
breaks  in,  125;  motor,  175;  control, 
175;  with  inductance  and  resistance, 
242 

Cleanliness,  326 

Clockwise  rotation,  of  separately 
excited  dynamo,  122;  of  shunt 
dynamo,  122;  of  series  dynamo,  123; 
of  compound  dynamo,  123;  of 
series  motor,  146;  of  shunt  motor, 
146,  147;  connection  of  single- 
phase  motor  for,  309 

Closed  circuit  characteristic,  88 

Closed-coil  armature,  75 

Coil,  225;  primary,  227;  secondary, 
227;  choking,  238,  304;  without 
iron,  vector  diagram  of,  240 

Commercial  efficiency  of  a  dynamo,  130 

Commutator,  simple,  71 

Commutator  motors,  281 

Compound  dynamos,  97;  connections, 
98,  ready  for  switching  in  parallel, 
199, 200 

Compound  motor,  143 

Concentric  cables,  43 

Condenser,  306 

Conductor  in  a  magnetic  field,  65 

Conductors  of  electricity,  21 

Connecting,  in  parallel,  29;  in  series, 
29 

Connection,  correct  and  incorrect, 
for  braking  motor,  176;  earth,  329 

Connection  box,  174 

Connections,  for  booster  when  charg- 
ing, 189;  of  single-phase  motor,  309; 
star,  317;  mesh  or  delta,  320 

Consequent  \  oles,  109 

Constant  current,  71 

Contactors,  172,  178 

Continuous  current,  71,  72 

Continuous-current  dynamo,  75 

Control  cable,  174 


INDEX 


341 


Control  couplers,  174 

Control  cut-out  switch,  174 

Control  fuses,  174 

Control  rheostat,  174 

Controller,  street-car,  cylinder  develop- 
ment of,  168;  master,  173,  178 

Converters,  rotary,  249,  276 

Cooper-Hewitt  lamp,  216 

Copper  required  to  carry  energy  three- 
phase  and  single-phase,  321 

Copper  wire,  data  on,  54,  55 

Copper  wires  and  cables,  table  of 
sizes,  resistances,  and  maximum 
currents  of,  53 

Core,  2;   iron,  225,  227 

Core  loss  of  a  dynamo,  131 

Correct  connection  for  braking  motor, 
176 

Coulomb,  15 

Counter-clockwise  rotation,  of  sepa- 
rately excited  dynamo,  122;  of 
shunt  dynamo,  122;  of  series  dy- 
namo, 123;  of  compound  dynamo, 
123;  of  series  motor,  145;  of  shunt 
motor,  146;  connections  of  single- 
phase  motor  for,  309 

Counter-electro-motive  force,  134 

Couplers,  control,  174 

Crater,  208 

Current,  electric,  2,  10,  60;  measure- 
ment of,  14;  unit,  16;  maximum, 
47;  induced,  66,  67,  226;  constant, 
71;  rectified  or  continuous,  71,  72; 
for  exciting  magnet  coils,  86;  run- 
ning light,  131;  effective  or  virtual, 
224;  inducing  or  primary,  226;  watt- 
less, 237,  307;  field,  curve  of,  258, 
261;  at  no  load,  274,  275;  multi- 
phase alternating,  283;  quarter- 
phase  or  two-phase,  287;  rotary  or 
three-phase,  290,  292;  magnetizing, 
293;  no-load,  294;  charging,  305; 
multiphase,  transmission  of,  314; 
phase ,  323 

Current  indicator,  194 

Current  strength,  23,  29 

Current  transformers,  327 

Currents,  Eddy,  77,  78 

Curve  of  field  current  plotted  against 
load,  258,  260 

Curve  of  voltage  with  load  variation  of 
an  alternator,  vector  diagram  of, 
259 

Curves,  magnetic,  8;  saturation,  87; 
sine,  218 

Cut-put,  47,  48;  for  large  current,  48; 
minimum,  192;  maximum,  193 


Cycles  of  a  dynamo,  219 
Cylinder   development    of    street-car 
controller,  168 

Damping,  211 

Dead  points  in  working  of  bicycle  and 
sewing-machine ,  303 

Decomposition  of  water,  3 

Delta  connections,  320 

Deprez  instrument,  56,  57,  133;  used 
as  an  ammeter,  57;  used  as  a  pole- 
finder,  58;  used  as  a  voltmeter,  58, 
202 

Diagrams,  vector,  239,  259,  260,  261 

Difference  of  electrical  potential,  4 

Differential  arc  lamp,  210 

Direct-current  dynamos  in  parallel, 
198 

Direct-current  motors,  operating  troub- 
les with,  178 

Direction  of  rotation,  methods  for 
changing,  121 

Direction  of  rotation  of  a  motor,  145 

Discharging,  180,  194 

Discs,  toothed,  83 

Double-cell  switch,  188,  191 

Double-current  alternator,  248 

Double-pole  throw-over  switch,  150 

Driving-keys,  83 

Drop  of  potential,  27;  to  reduce,  28 

Drum  armature,  79;  connections,  79, 
81,  82,  83;  wound,  84;  without 
winding,  84;  four-pole  parallel,  113; 
four-pole  series,  114 

Drying,  326 

Dynamo,  68,  133;  bipolar,  71;  con- 
tinuous-current, 75;  shunt,  92; 
self-excitation  of,  92;  series,  94; 
compound,  97;  types  of,  98;  over- 
compounded,  98;  Edison,  99;  Kapp, 
99;  C  type  of,  100;  Manchester, 
100;  Lahmeyer,  101;  Gramme,  102; 
multipolar,  105;  four-pole,  106; 
six-pole,  107;  two-coil  four-pole, 
108,  109;  non-excitation  of,  124; 
commercial  efficiency  of,  129,  130; 
direct-current,  198;  switching,  201; 
cycles  of  a,  219 

Dynamo  armature,  74,  133 

Dynamo-brushes,  material  for,  120 

Dyne,  9 

Earth  connection,  329 

Eddy  currents  in  iron,  77,  78 

Edison,  Thomas  A.,  186;  dynamo,  99; 

battery,   186;     carbon    lamp,     203; 

lamp-holder,  204 


342 


INDEX 


Effective  current,  224 

Effective  voltage,  225 

Effective  watts,  246 

Efficiency,  of  dynamos,  129;  stray 
power  method  of  getting,  131;  of 
the  accumulator,  183;  of  an 
alternator,  257 

Electric  brake,  175 

Electric  current,  2;  influence  of,  on 
a  magnetic  needle,  10 

Electric  lighting,  203 

Electric  mains,  40 

Electric  motor,  133 

Electric  traction,  164 

Electric  units,  14,  38 

Electrical  energy,  197;  the  kilowatt- 
hour  a  commercial  unit  for,  197 

Electrical  machines,  68 

Electrical  output,  39 

Electrical  phenomena,  1 

Electrical  potential,  difference  of,  4 

Electrical  power,  34,  197;  the  watt 
a  convenient  unit  for,  197 

Electrical  pressure,   motion   essential 
for  maintenance  of,  66 

Electrically  driven  fan,  164 

Electro-dynamic  action,  60 

Electro-dynamometers,  60,  61 

Electrolytic  polarization,  179 

Electro-magnetic  ammeter,  18,  19 

Electro-magnets,  2,  61;  horseshoe,  63 
64;   straight  or  bar,  63,  64 

Electro-motive  force,  3  4,  27;  back, 
134,  226;  of  self-induction,  235, 
236;  of  an  alternator,  253 

Electro-motor,  133 

Enclosed  arc  lamp,  214.  216 

Enclosed  fuse,  49 

Enclosed  motor,  159,  160 

Enclosed  regulating  resistance ,  90 

Energy,  transformation  of,  129;  chemi- 
cal, 197;  electrical,  197 

Engineering,  electrical,  fundamental 
principles  of,  1 

Equalizing  wire,  199 

Equivalence  of  electrical  mechanical 
and  heating  effects,  37 

Examples  of  installation  calculations, 
46 

Expulsion  fuse-block,  49 

Expulsion-tube  fuse-block,  49 

Fahrenheit,  24 

Fan,  electrically  driven,  164 

Faraday,  14 

Faure,  181 

Ferranti  transformer,  231,  232 


Ferraris,  283     . 

Field,  magnetic,  9,  234,  235,  286,  287; 
pulsating,  283,  303;  rotating,  283 

Field  magnets,  material  for,  102 

Field  of  magnetic  force,  9 

Field  rheostats,  91 

Filaments  for  incandescent  lamps, 
205-207 

Filings  round  current,  arrangement 
of,  13 

Fixed  lead,  121 

Foot-pounds,  38 

Force,  electro-motive,  3,  4,  27,  235, 
236,  253;  exerted  by  a  magnetic 
pole,  6;  magnetic,  lines  and  field 
of,  8,  9;  lines  of,  12,  13,  56,  60, 
75,  104;  magnetizing,  15, 17;  mag- 
neto-motive, 15,63;  back  or  counter- 
electro-motive,  134,  226 

Former-wound  armature,  114, 115, 117 

Forming,  180 

Four-pole  three-phase  motors,  292 

Four-pole  two-phase  motor,  288 

Friction  loss  of  a  dynamo,  130,  131 

Fundamental  principles  of  electrical 
engineering,  1 

Fuse,  47,  48;  for  large  current,  48; 
enclosed,  49;  plug,  52;  control, 
174;  high-tension,  328 

Fuse-block,  expulsion,  49;  expulsion- 
tube,  49 

Galvanic  battery,  1,  2 

Galvanic  cell,  1 

Galvanometer,  73;   simple,  11 

Galvanometer  coil,  11 

Gas  voltameter  18 

Generator,  133,  250;  alternating-cur- 
rent, 247;  motor,  276,  277,  278 

Glow  lamps,  35,  203;  brass  cap  of, 
204;  star-connected,  319;  on  one 
phase,  319 

Gramme,  74 

Gramme  armature,  74,  320 

Gramme  dynamo,  102 

Gramme  ring,  247,  277 

Hand  rule,  67 

Heating  effect,  35,  37 

Heating  of  a  line   47 

Hefner-Alteneck,  82 

Helix,  lines  of  force  of,  13;   resultant 

field  of,  13 
Henry,  242 
High  tension,  326 
High-tension  fuse,  328 
High-tension  insulator,  42 


INDEX 


343 


High-tension  plants,  safety  in,  328 

High-tension  switch,  328 

High  voltages,  46 

Holder,  bayonet,  204;   Swan,  204 

Horse-power,  38 

Horseshoe  magnet,  5 

Hot-wire  ammeter,  19,  20 

Hot-wire  voltmeter,  34 

Hydraulic  analogy,  3 

Hysteresis,   104 

Hysteresis  loss,  129,  130 

Incandescent  lamp,  35,  203;  the 
Nernst,  205-207 

Incorrect  connection  for  braking 
motor,  176 

Induced  current,  law  of,  14;  direction 
of,  66,  67,  226;  rule  for  determining, 
67 

Inducing  or  primary  current,  curve  of, 
226 

Induction,  65 

Induction  motor,  283;  two-phase,  285; 
actions  in,  293;  input  and  output 
of,  295,  298;  formulae  for  torque  of, 
296,  297;  curve  of  torque  of,  296; 
formulae  for  horse-power  of,  297; 
curves  showing  efficiency,  maximum 
output,  etc.,  of,  298;  power  factor 
of,  298;  efficiency  of,  298;  advan- 
tages of,  299;  vector  diagram  of, 
301;  single-phase,  302 

Inductionless  load,  322 

inductor  alternator,  Oerlikon,  263 

Inductor  machine,  armature  and  ex- 
citing bobbin  of,  262 

Influence  of  electric  currents  on  each 
other,  60 

Installation  calculations,  46 

Installation  of  Type  M  Control,  175 

Insulation,  327-329 

Insulator,  21;  high-tension,  42;  por- 
celain, 42 

Interlinked  phases,  315;  voltage  be- 
tween, diagram  of,  316 

Internal  resistance,  27 

Iron  core,  diagrams  of,  225,  227 

Iron  loss  of  a  dynamo,  131 

Joule,  39 

Kapp  type  of  dynamo,  99 
Kilowatt-hour,  195,  197 
Krizik  arc  lamp,  211,  212 

L,  use  of,  241 
Lag,  237 


Lahmeyer  type  of  dynamo,  101 

Lamp-holder,  204 

Lamps,    incandescent,     35,     203;     in 

series  and  parallel,  45;  testing,    126; 

carbon,  203;    new  types    of,     216; 

synchronizing,  265 
Law,    of    induced     currents,    14;     of 

Ohm,  23,  26;  of  change  of  resistance 

with  temperature,  24;    Lena's.,  67, 

274,  284 

Lead,  fixed,  121;   of  current,  307 
Lenz's  law,  67,  274,  284 
Lighting,  electric,  203 
Lightning  arresters,  330 
Line,  heating  of  a,  47 
Lines,  stray,  64 
Lines  of  force,  60,  104;  round  current, 

12;     of    helix,    13;     magnetic,    56; 

through  ring  armature,  75 
Lines  of  magnetic  force,  8;   meanings 

of,  8,  9 

Load,  inductionless,  322 
Locomotive  motor  armature,  85 
Loop  winding,  112 
Loss  in  field  windings,  130 
Losses  in  transformation  of  energy,  129 

fJL,  use  of,  17 

Machines,  electrical,  68;  magneto- 
electric,  73,  247;  for  charging 
accumulators,  187 

Magnet,  bar,  5;  horseshoe,  5;  poles 
of,  13,  62;  influence  of  a,  on  an 
electric  current,  56;  molecular,  62; 
field,  102 

Magnet  frame,  eighteen-pole,  108 

Magnet  system,  85;  of  Oerlikon  in- 
ductor alternator,  264 

Magnetic  blow-out,  177 

Magnetic  circuit,  15,  63;  of  bipolar 
dynamo,  86;  four-pole.  106;  four- 
pole  inter-pole,  1.3 

Magnetic  curves,  8 

Magnetic  field,  9,  286,  287;  alteration 
of,  234,  235 

Magnetic  lines  of  force,  56 

Magnetic  needle,  5,  6;    pivoted.  5 

Magnetism,  unit  of,  9;  residual,  62; 
permanent,  63 

Magnetite  arc  lamp,  215,  216 

Magnetization  characteristic,  87 

Magnetizing  current  of  a  motor,  293 

Magnetizing  force,  15,  17 

Magneto,  73,  127 

Magneto-electric  machine,  73,  247 

Magneto-generator,  68 

Magneto-motive  force,  15,  63 


344 


INDEX 


Magnets  and  magnetic  lines  of  force, 
56 

Main  circuit,  92 

Mains,  electric,  40 

Manchester  type  of  dynamo,  100 

Master  controller,  173,  178 

Master  switch,  173 

Maximum  current,  47 

Maximum  cut-outs,  193 

Measurement  of  currents,  14 

Measuring  transformers,  327 

Mesh  connections,  320,  321 

Metal  ring,  repulsion  of,  225 

Metric  system,  24 

Minimum  cut-outs,  192 

Minium,  181 

Molecular  magnets,  62 

Molecules,  62 

Motor,  electric,  133;  shunt,  136; 
speed  regulation  of,  137;  series,  139, 
176;  compound,  143;  lor  certain 
purposes,  159;  enclosed,  159,  160; 
connected  to  machine  tool,  160; 
connected  to  pump,  161;  commu- 
tating  pole,  161;  connected  to  ele- 
vator, 162;  connected  to  mine 
hoist,  163;  street-car,  165;  braking, 
176;  direct-current,  178;  alternat- 
ing-current, 270,  300,  308,  311; 
synchronous,  270;  polyphase  syn- 
chronous, 273,  274;  commutator, 
281;  asynchronous,  285;  two- 
phase,  288;  three-phase,  292,  295; 
single-phase,  304.  See  also  Induc- 
tion motor 

Motor  armature,  133 

Motor  generator,  276,  277,  278 

Motor  starting  switch,  154 

Moving  boat,  action  of,  66 

Moving  coil  ammeter,  construction  of, 
57 

Multiphase  alternating  current    283 

Multiphase  currents,  transmission  of, 
314 

Multiple-unit  control  system,  167 

Multipolar  dynamos,  105 

Needle,  magnetic,  5,  6 

Negative  pole,  3 

Nernst,  205;   lamp,  205-207 

Neutral  point  in  three-phase  wind- 
ings, 320 

Neutral  zone,  76 

No-load  current,  294 

Non-excitation  of  dynamos,  causes  of, 
124 

North  and  south  poles  of  magnet,  13, 62 


Number  of  amperes  passing  through  a 
circuit,  23 

a),  use  of,  22 

Oerlikon  inductor  alternator,  magnet 

system  of,  263;    lines  of  force  in,  263 
Ohm,  22 
Ohm's  Law,   23,  26;     for  magnetism, 

64 
Operating  troubles  with  direct-current 

motors,  178 
Osmium  as  a  filament  for  glow  lamps, 

216 

Output,  electrical,  39 
Output  of  a  dynamo,  103 
Overload,  140 
Oxide  of  iron,  179 

TT,  use  of,  10 

Paper,  pole-finding,  202 

Parallel  winding,  112 

Period,  284 

Permanent  magnetism,  63 

Permeability,  17 

Phase,  quantities  out  of,  240;    main, 

304,  307;   auxiliary,  304,  307 
Phase  current,  323 

Phase  -difference,  233;  caused  by 
self-induction,  304;  caused  by  ca- 
pacity, 305;  diagrams  of  capacity, 

305,  306 

Phase  meters,  246 
Phases,  interlinked,  315 
Phenomena,  electrical,  1 
Pilsen  lamp,  211 
Pitch,  80 

Pivoted  magnetic  needle,  5 

Plante",  180;  plates,  180;  accumu- 
lators, 181 

Plates,  Plante,  180 

Plot  of  current  at  no  load,  274,  275 

Plug  fuse,  52 

Point,  neutral,  320 

Polarity,  6;   right,  201,  202 

Polarization,  electrolytic.  179 

Pole-finder,  58 

Pole-finding  paper,  202 

Poles,  negative  and  positive,  3:  north 
and  south,  of  magnet,  13,  62;  con- 
sequent, 109 

Pole-shoes,  68 

Polyphase  synchronous  motor,  one 
phase  of,  273,  274 

Porcelain  insulator,  42 

Portable  storage  battery,  185 

Positive  pole,  3 

Potential,  electrical,  4;   drop  of,  27 


INDEX 


345 


Power,  electrical,  34,  197 

Power  factor,  241,  246 

Power  loss,  44,  45 

Power  transmission,  series  method  of, 

141 

Presspahn  tubes,  251 
Pressure,  electrical,  66 
Pressure  losses,  41 
Primary    armature,    285;     ready    for 

winding,    289;     completely   wound, 

289 

Primary  current,  226 
Prone y  brake,  295 
Properties  of  angles   concerned  with 

alternating  currents,  217 
Pulsating  field,  283,  303 

Quantities  out  of  phase,  240 
Quarter-phase  alternator,  255 
Quarter-phase  current,  287 

Raymond,  241,  297 

Reactance,  synchronous,  260 

Real  watts,  246 

Rectified  current,  71,  72 

Regulating  resistance,  89;  enclosed,  90 

Regulator,  shunt,  127,  128,  157 

Remanence,  62 

Residual  magnetism,  62 

Resistance,  23,  25,  26;  specific,  23; 
calculation  of,  23;  internal,  27; 
resultant,  31;  regulating,  89 

Resistance  frames,  25 

Resistance  of  a  wire,  24 

Resistances  in  parallel,  30 

Resultant  resistance,  31 

Resultant  voltage,  323 

Reverser,  172 

Reversing  and  starting  switch,  152, 
157,  158 

Reversing  apparatus,  150 

Reversing  of  alternating  -  current 
motors,  308 

Revolutions,  104 

Rheostat,  control,  174 

Right  polarity,  two  methods  of  se- 
curing, 201,  202 

Ring,  metal,  225;    Gramme    247,  277 

Ring  armature,  74;  lines  of  force 
through,  75;  with  commutator,  76, 
77;  two-pole,  118;  four-pole  paral- 
lel, 110,  111;  four-pole  series,  112, 
motor,  134;  with  slip-rings,  247 

Ring  transformer,  228 

Rotary  converters  or  rotaries,  249,  276 

Rotary  or  three-phase  current,  292 

Rotating   field,    283;     production    of, 


diagrams,  283,  284,  290,  291;  speed 

of,  288 
Rotation,    direction   of,   methods    for 

changing,  121;  of  a  motor,  145.     See 

also  Clockwise  and  Counter-clockwise 
Rotor,  285;   squirrel-cage,  286;    speed 

of,  288;   wound,  289,  295;   triphase, 

299 

Running  light  current,  131 
Running  position  of  series  motor,  176 

Safety  in  high-tension  plants,  328 

Saturation,  condition  of,  65 

Saturation  curve,  87 

Search-light,  215 

Secondary  winding,  226 

Self-excitation,  91;  of  dynamos,  92 

Self-induction, 153,  226;  electro-motive 
force  of,  235,  236 

Series  arc  lamp,  209 

Series  circuit,  29 

Series  dynamos,  94;  connections,  95; 
closed  circuit  characteristic  of,  96; 
working  in  parallel,  199,  200 

Series  method  of  power  transmission, 
141 

Series  motor,  139;  with  starting  resist- 
ance, 139;  speed  and  torque  curves 
of,  142;  counter-clockwise  rotation, 
145;  clockwise  rotation,  146 

Series  winding,  95,  111,  113 

Sewing-machine,  diagrams  of  working 
of,  303 

Shell  transformer,  231 

Short-circuit  armature,  295 

Short-circuiting,  326 

Shunt,  58 

Shunt  arc  lamp,  210 

Shunt  circuit,  92 

Shunt  dynamos,  92;  connections,  92; 
external  characteristic  of,  93,  94; 
ready  for  switching  in  parallel,  198, 
199, 200 

Shunt  for  ammeter,  59 

Shunt  motor,  with  starting  resistance, 
136;  with  starting  resistance  and 
shunt  regulator  for  speed  regulation, 
138;  clockwise  rotation,  146,  147; 
counter-clockwise  rotation,  146; 
with  wrong  connection,  147,  148; 
rule  for  connecting  up,  148;  with 
change-over  switch,  151 

Shunt  regulator,  157;  automatic,  127, 
128 

Siemens,  Werner,  92;  armature,  69, 
74,  75,  84,  221,  248,  270;  lamp- 
holder,  204 


346 


INDEX 


Simple  circuit,  27 

Simple  commutator,  71;  second  posi- 
tion of,  71;  third  position  of,  72 

Simple  galvanometer,  11 

Sine  curve  of  electro-motive  force  and 
current,  218;  90°  apart  in  phase, 
220 

Single-phase  induction  motor,  302; 
with  auxiliary  phase  with  self-in- 
duction, 304;  with  auxiliary  phase 
having  capacity,  307 

Slip,  285,  300 

Slip-ring  armatures,  293 

Slip-rings,  70,  310 

Slots,  different  shapes  of,  251 

Smooth  armatures,  83 

Solenoid,  12 

Spark,  break,  222 

Sparking  and  displacement  cf  brushes, 
118 

Sparking  with  starters  and  shunt 
regulators,  153 

Specific  resistance ;  23 

Speed  and  torque  curves  of  series 
motor,  142 

Speed-measuring  devices,  mechanical, 
265;  electrical,  265 

Speed  regulation  of  a  motor,  137 

Spiral  without  self-induction,  245 

Sprague-General  Electric  Type  M 
Control,  169 

Square,  to  determine  area  of,  36 

Squirrel-cage  armatures,  293 

Squirrel-cage  rotor,  286 

Squirrel-cage  winding,  274 

Standard  Wire  Gauge,  22 

Star  connections,  317,  321 

Star  method  of  arrangement  of  phases 
of  three-phase  system,  320 

Starter,  166;  with  inductionless  break, 
having  shunt  slip-ring,  154;  without 
slip-ring,  156 

Starting  and  reversing  switch  for 
shunt  motor,  152,  157 

Static  transformer,  278 

Stationary  state,  40 

Stator,  285 

Steel,  cast,  103 

Steinmetz,  Charles  P.,  104,  259,  260 

Storage  cells,  184 

Stray  lines,  64 

Stray  power  method  of  getting  effi- 
ciency, 131 

Street-car  controller,  166-168 

Street-car  motor,  closed,  165;  open, 
165 

Sum,  vector,  240 


Swan,  203;    holder,  204 

S.  W.  G.,  22 

Switch,  double-pole  throw-over  or  two- 
pole  change-over,  150,  151;  revers- 
ing and  starting,  152,  157,  158; 
motor  starting,  154;  master,  173; 
control  cut-out,  174;  cell,  186; 
double-cell,  188,  191;  battery,  189, 
190;  high-tension,  328 

Switchboards,  334;  astatic  instrument 
for,  59;  high-tension,  327;  insu- 
lation of,  329 

Switching  dynamos  in  parallel,  201 

Switching  in  parallel  of  alternating- 
current  machines,  265 

Synchronism,  265 

Synchronizer,  265;  for  multiphase 
machines,  324 

Synchronizing  action,  269 

Synchronizing  lamps,  connections,  265; 
cross-connected,  266;  for  high-ten- 
sion circuits,  267;  arrangement  of, 
for  three-phase  circuit,  324;  for  high- 
tension  three-phase  circuits,  325 

Synchronous  motors,  270;  disad- 
vantages of ,  271 ;  advantages  of,  272 ; 
electro-motive  forces  of,  272;  field 
insulation  of,  275;  use  of,  276; 
starting,  277 

Synchronous  reactance,  260 

Synchroscope,  Westinghouse,  265 

System,  metric,  24;  Ward-Leonard, 
158;  multiple-unit  control,  167; 
train  control,  171;  two-phate,  314; 
three-phase,  316 

Temperature,  change  of  resistance 
with,  24 

Tension,  high,  326 

Testing  lamp,  126 

Three-phase  alternator,  256,  334 

Three-phase  current,  290 

Three-phase  motor,  wound  rotor  of, 
295 

Three-phase  system,  with  six  mains, 
316;  voltage  between  outer  ter- 
minals, 316;  with  four  mains,  317; 
with  three  mains,  317;  current  in 
common  return,  318;  power  in,  322 

Toothed  armatures,  83 

Toothed  discs,  open  slots,  83;  nearly 
closed  slots,  83 

Traction,  electric,  164 

Train  control  system-  171 

Transformation  of  energy,  129 

Transformers,  227;  ring,  228;  shape 
of,  229;  with  horseshoe-shaped 


INDEX 


347 


iron  core,  230;  with  coils  sub- 
divided, 230;  with  coils  wound  one 
on  the  other,  231;  shell,  231; 
Ferranti  type,  231,  232;  applica- 
tions of,  232;  vector  diagram  of, 
240;  regulation  of,  262;  static,  278; 
measuring,  327 

Triphase  rotor,  299 

Tri-phaser,  forty-pole  armature  of,  313 

Trolley,  165 

Tubes,  Presspahn,  251 

Two  -phase  current,  287;    motor,  286 

Two-phase  induction  motor,  285,  286; 
ready  for  winding,  289;  wound, 
289 

Two-phase  system,  with  four  mains, 
314;  with  three  mains,  315 

Two-pole  change-over  switch,  150,  151 

Two -pole  dynamo,  71 

Type  C  motor  complete,  289 

Type  M  Control,  169;  automatic 
features  of,  175 

Types  of  dynamos,  98 

Unit,  of    magnetism,    9;     of    electric 

pressure,  14 
Unit  current,  16 
Units,  24;    electric,  14,  38 

Value  of  resistances  in  parallel,  30 
Vector  diagram  method,  221 
Vector  diagrams,  239,  259,  260,  261 
Vector  sum,  240 
Virtual  current,  224 
Virtual  voltage,  225 
Volta,  18 


Voltage,  14;  high,  46;  effective  or 
virtual,  225;  square  root  of  mean 
square,  243;  curve  of,  259;  in 
three-phase  rotary,  279:  resultant, 
323 

Voltage  drop,  44 

Voltameter,  18 

Volt-ampere,  35 

Voltmeter,  electro-magnetic  type,  33, 
202;  hot-wire,  34;  for  cell  testing, 
194;  switch,  202 

Ward-Leonard  system  of  control  for 
shunt  or  series  motors,  158 

Water,  decomposition  of,  3 

Water  current,  production  of,  4 

Watt,  35,  197 

Watt,  James,  38 

Wattless  current,  237,  307 

Wattmeter,  60,  244 

Watts,  apparent,  246;  effective  or 
real,  246 

Wave  winding,  113 

Westinghouse  lightning  arrester,  331 

Westinghouse  synchroscope,  265 

Weston  ammeter,  58 

Winding,  series,  95,  111,  113;  loop, 
112;  parallel,  112;  wave,  113; 
field,  130;  secondary,  226;  squirrel- 
cage,  274;  three-phase,  320 

Wire,  resistance  of  a,  24;  equalizing, 
199 

Working  of  direct-current  dynamos  in 
parallel,  198 

Zone,  neutral,  76 


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ca 8vo,    5  00' 

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^»i*»nagc I2mO,       I    OO- 

Graves's  Forest  Mensuration 8vo,  4  oo 

Green's  Principles  of  American  Forestry I2mo,  i  50 

Grotenfelt's  Principles  of  Modern  Dairy  Practice.     (Woll.) i2mo,  2  oo- 

Kemp's  Landscape  Gardening I2mo,  2  50- 

Maynard's  Landscape  Gardening  as  Applied  to  Home  Decoration I2mo,  i  50- 

*  McKay  and  Larsen's  Principles  and  Practice  ot  Butter-making 8vo,  i  sa 

Sanderson's  Insects  Injurious  to  Staple  Crops i2mo,  i  50 

Insects  Injurious  to  Garden  Crops.     (In  preparation.) 

Insects  Injuring  Fruits.     (In  preparation.) 

Stockbridge's  Rocks  and  Soils 8vo,  2  50 

Winton's  Microscopy  of  Vegetable  Foods 8vo,  7  50 

Woll's  Handbook  for  Farmers  and  Dairymen i6mo,  i  50 


ARCHITECTURE. 

Baldwin's  Steam  Heating  for  Buildings I2mo,  2  50 

Bashore's  Sanitation  of  a  Country  House I2mo.  i  oo 

Berg's  Buildings  and  Structures  of  American  Railroads 4to,  5  oo 

Birkmire's  Planning  and  Construction  of  American  Theatres 8vo,  3  oo 

Architectural  Iron  and  Steel. 8vo,  3  50 

Compound  Riveted  Girders  as  Applied  in  Buildings 8vo,  2  oo 

Planning  and  Construction  of  High  Office  Buildings . 8vo,  3  50 

Skeleton  Construction  in  Buildings 8vo,  3  oo 

Brigg's  Modern  American  School  Buildings 8vo,  4  oo 

1 


SHORT-TITLE     CATALOGUE 

OF  THE 

PUBLICATIONS 

OF 

JOHN   WILEY   &    SONS, 

NEW  YORK, 
LONDON:  CHAPMAN  &  HALL,  LIMITED. 


ARRANGED  UNDER  SUBJECTS. 


Descriptive  circulars  sent  on  application.      Books  marked  with   an  asterisk  (*)  are  sold 
at   net  prices   only.       All  books  are  bound  in  cloth  unless  otherwise  stated. 


AGRICULTURE. 

Armsby's  Manual  of  Cattle-feeding 12010,  Si  75 

Principles  of  Animal  Nutrition 8vo,  4  oa 

Budd  and  Hansen's  American  Horticultural  Manual: 

Part  I.  Propagation,  Culture,  and  Improvement I2mo,  50 

Part  II.  Systematic  Pomology i2mo,  50 

Downing's  Fruits  and  Fruit-trees  of  America 8vo,  oo< 

Elliott's  Engineering  for  Land  Drainage i2mo,  so- 
Practical  Farm  Drainage I2mo,  oo 

Graves's  Forest  Mensuration 8vo,  oo 

Green's  Principles  of  American  Forestry I2mo,  50 

Grotenfelt's  Principles  of  Modern  Dairy  Practice.     (Woll.) i2mo,  oa 

Kemp's  Landscape  Gardening i2mo,  50- 

Maynard's  Landscape  Gardening  as  Applied  to  Home  Decoration i2mo,  50 

*  McKay  and  Larsen's  Principles  and  Practice  ot  Butter-making 8vo,  50 

Sanderson's  Insects  Injurious  to  Staple  Crops I2mo,  50- 

Insects  Injurious  to  Garden  Crops.     (In  preparation.) 
Insects  Injuring  Fruits.     (In  preparation.) 

Stockbridge's  Rocks  and  Soils 8vo,  2  50 

Winton's  Microscopy  of  Vegetable  Foods 8vo,  7  50 

Woll's  Handbook  for  Farmers  and  Dairymen i6mo,  i  50 


ARCHITECTURE. 

Baldwin's  Steam  Heating  for  Buildings I2mo,  2  50 

Bashore's  Sanitation  of  a  Country  House i2mo.  i  oo 

Berg's  Buildings  and  Structures  of  American  Railroads 4to,  5  oo 

Birkmire's  Planning  and  Construction  of  American  Theatres 8vo,  3  oo 

Architectural  Iron  and  Steel. 8vo,  3  50 

Compound  Riveted  Girders  as  Applied  in  Buildings 8vo,  2  oo 

Planning  and  Construction  of  High  Office  Buildings , 8vo,  3  50 

Skeleton  Construction  in  Buildings 8vo,  3  oo 

Brigg's  Modern  American  School  Buildings 8vo,  4  oo 

1 


Carpenter's  Heating  and  Ventilating  of  Buildings Svot  4  oo 

Freitag's  Architectural  Engineering 8vo,  3  50 

Fireproofing  of  Steel  Buildings 8vo,  2  50 

French  and  Ives's  Stereotomy 8vo,  2  50 

Gerhard's  Guide  to  Sanitary  House-inspection .-. . i6mo,  i  oo 

Theatre  Fires  and  Panics I2mo,  i  50 

*Greene's  Structural  Mechanics 8vo,  2  50 

Holly's  Carpenters'  and  Joiners'  Handbook i8mo,  75 

Johnson's  Statics  by  Algebraic  and  Graphic  Methods 8vo,  2  oo 

Kidder's  Architects' and  Builders' Pocket-book.  Rewritten  Edition.  i6mo,mor.,  5  oo 

Merrill's  Stones  for  Building  and  Decoration 8vo,  5  oo 

Non-metallic  Minerals:   Their  Occurrence  and  Uses 8vo,  4  oo 

Monckton's  Stair-building 4to,  4  oo 

Patton's  Practical  Treatise  on  Foundations 8vo,  5  oo 

Peabody's  Naval  Architecture 8vo,  7  50 

Rice's  Concrete-block  Manufacture 8vo,  2  oo 

Richey's  Handbook  for  Superintendents  of  Construction i6mo,  mor.,  4  oo 

*              Building  Mechanics'  Ready  Reference  Book.     Carpenters'  and  Wood- 
workers' Edition i6mo,  morocco,  i  50 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish 8vo,  3  oo 

Siebert  and  Biggin's  Modern  Stone-cutting  and  Masonry 8vo,  i  50 

Snow's  Principal  Species  of  Wood 8vo,  3  50 

Sondericker's  Graphic  Statics  with  Applications  to  Trusses,  Beams,  and  Arches. 

8vo,  2  oo 

Towne's  Locks  and  Builders'  Hardware i8mo,  morocco,  3  oo 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  oo 

Sheep,  6  50 

Law  of  Operations  Preliminary  to  Construction  in  Engineering  and  Archi- 
tecture  8vo,  5  oo 

Sheep,  5  50 

Law  of  Contracts 8vo,  3  oo 

Wood's  Rustless  Coatings:   Corrosion  and  Electrolysis  of  Iron  and  Steel.  .8vo,  4  oo 
Worcester  and  Atkinson's  Small  Hospitals,  Establishment  and  Maintenance, 
Suggestions  for  Hospital  Architecture,  with  Plans  for  a  Small  Hospital. 

i2mo,  i  25 

The  World's  Columbian  Exposition  of  1893 Large  4to,  i  oo 


ARMY  AND  NAVY. 

Bernadou's  Smokeless  Powder,  Nitro-cellulose,  and  the  Theory  of  the  Cellulose 

Molecule ' i2mo,  2  50 

*  Bruff's  Text-book  Ordnance  and  Gunnery 8vo,  6  oo 

Chase's  Screw  Propellers  and  Marine  Propulsion 8vo,  3  oo 

Cloke's  Gunner's  Examiner 8vo,  i  50 

Craig's  Azimuth 4to,  3  50 

Crehore  and  Squier's  Polarizing  Photo-chronograph 8vo,  3  oo 

*  Davis's  Elements  of  Law 8vo,  2  50 

*  Treatise  on  the  Military  Law  of  United  States 8vo,  7  oo 

Sheep,  7  50 

De  Brack's  Cavalry  Outposts  Duties.     (Carr.) 24mo,  morocco,  2  oo 

Dietz's  Soldier's  First  Aid  Handbook i6mo,  morocco,  i  25 

*  Dudley's  Military  Law  and  the  Procedure  of  Courts-martial. .  .  Large  i2mo,  2  50 
Durand's  Resistance  and  Prdpulsion  of  Ships 8vo,  5  oo 

*  Dyer's  Handbook  of  Light  Artillery i2mo,  3  oo 

Eissler's  Modern  High  Explosives 8vo,  4  oo 

*  Fiebeger's  Text-book  on  Field  Fortification Small  8vo,  2  oo 

Hamilton's  The  Gunner's  Catechism i8mo,  i  oo 

*  Hofif's  Elementary  Naval  Tactics 8vo,  i  50 

2 


Ingalls's  Handbook  of  Problems  in  Direct  Fire 8vo,  4  oc 

*  Ballistic  Tables 8vo,  i  50 

*  Lyons's  Treatise  on  Electromagnetic  Phenomena.  Vols.  I.  and  II.  .8vo,  each,  6  oo 

*  Mahan's  Permanent  Fortifications.    (Mercur.) 8vo,  half  morocco,  7  So 

Manual  for  Courts-martial i6mo,  morocco,  i  50 

*  Mercur's  Attack  of  Fortified  Places I2mo,  2  oo 

*  Elements  of  the  Art  of  War 8vo,  4  oo 

Metcalf's  Cost  of  Manufactures — And  the  Administration  of  Workshops.  .8vo,  5  oo 

*  Ordnance  and  Gunnery.     2  vols I2mo,  5  oo 

Murray's  Infantry  Drill  Regulations i8mo,  paper,  10 

Nixon's  Adjutants'  Manual 24mo,  i  oo 

Peabody's  Naval  Architecture 8vo,  7  50 

*  Phelps's  Practical  Marine  Surveying 8vo,  2  50 

Powell's  Army  Officer's  Examiner i2mo,  4  oo 

Sharpe's  Art  of  Subsisting  Armies  in  War i8mo,  morocco,  i  50 

*  Tupes  and  Poole's  Manual  of  Bayonet  Exercises  and    Musketry  Fencing. 

24mo,  leather,  50 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

Weaver's  Military  Explosives 8vo,  3  oo 

*  Wheeler's  Siege  Operations  and  Military  Mining 8vo,  2  oo 

Winthrop's  Abridgment  of  Military  Law I2mo,  2  50 

Woodhull's  Notes  on  Military  Hygiene. i6mo,  i  50 

Young's  Simple  Elements  of  Navigation i6mo,  morocco,  2  oo 

ASSAYING. 

Fletcher's  Practical  Instructions  in  Quantitative  Assaying  with  the  Blowpipe. 

I2mo,  morocco,  i  50 

Furman's  Manual  of  Practical  Assaying 8vo,  3  oo 

Lodge's  Notes  on  Assaying  and  Metallurgical  Laboratory  Experiments.  .  .  .8vo,  3  oo 

Low's  Technical  Methods  of  Ore  Analysis 8vo,  3  oo 

Miller's  Manual  of  Assaying i2mo,  i  oo 

Cyanide  Process i2mo,  i  oo 

Minet's  Production  of  Aluminum  and  its  Industrial  Use.     (Waldo.) i2mo,  2  50 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

Ricketts  and  Miller's  Notes  on  Assaying 8vo,  3  oo 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc.) 8vo,  4  oo 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 

Wilson's  Cyanide  Processes i2mo,  i  50 

Chlorination  Process i2mo,  i  50 

ASTRONOMY. 

Comstock's  Field  Astronomy  for  Engineers 8vo,  2  50 

Craig's  Azimuth 4to,  3  50 

Crandall's  Text-book  on  Geodesy  and  Least  Squares 8vo,  3  oo 

Doolittle's  Treatise  on  Practical  Astronomy 8vo,  4  oo 

Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  oo 

Merriman's  Elements  of  Precise  Surveying  and  Geodesy 8vo,  2  50 

*  Michie  and  Harlow's  Practical  Astronomy 8vo,  3  oo 

*  White's  Elements  of  Theoretical  and  Descriptive  Astronomy i2mo  oo 

BOTANY. 

Davenport's  Statistical  Methods,  with  Special  Reference  to  Biological  Variation. 

i6mo,  morocco,  i  25 

Thomd  and  Bennett's  Structural  and  Physiological  Botany i6mo,  2  25 

Westermaier's  Compendium  of  General  Botany.     (Schneider.) 8vo,  2  oo 


CHEMISTRY. 

*  Abegg's  Theory  of  Electrolytic  Dissociation.    (Von  Ende.) i2mo,  i  25 

Adriance's  Laboratory  Calculations  and  Specific  Gravity  Tables i2mo,  i   25 

Alexeyeff's  General  Principles  of  Organic  Synthesis.     (Matthews.) 8vo,  3  oo 

Allen's  Tables  for  Iron  Analysis 8vo,  3  oo 

Arnold's  Compendium  of  Chemistry.     (Mandel.).  .  .  . Small  8vo,  3  50 

Austen's  Notes  for  Chemical  Students i2mo,  i  50 

Bernadou's  Smokeless  Powder. — Nitro-cellulose,  and  Theory  of  the  Cellulose 

Molecule i2mo,  2  50 

*  Browning's  Introduction  to  the  Rarer  Elements 8vo,  i  50 

Brush  and  Penfield's  Manual  of  Determinative  Mineralogy 8vo,  4  oo 

*  Claassen's  Beet-sugar  Manufacture.     (Hall  and  Rolfe.) 8vo,  3  oo 

Classen's  Quantitative  Chemical  Analysis  by  Electrolysis.    (Boltwood.).  .8vo,  3  oo 

Cohn's  Indicators  and  Test-papers lamo,  2  oo 

Tests  and  Reagents 8vo,  300 

Crafts's  Short  Course  in  Qualitative  Chemical  Analysis.   (Schaeffer.).  .  .i2mo,  i  50 

*  Danneel's  Electrochemistry.     (Merriam.) I2mo,  i   25 

Dolezalek's  Theory  of  the   Lead  Accumulator   (Storage   Battery).        (Von 

Ende.) i2mo,  2  50 

Drechsel's  Chemical  Reactions.     (Merrill.) i2mo,  i  25 

Duhem's  Thermodynamics  and  Chemistry.     (Burgess.) 8vo,  4  oo 

Eissler's  Modern  High  Explosives 8vo,  4  oo 

Effront's  Enzymes  and  their  Applications.     (Prescott.) 8vo,  3  oo 

Erdmann's  Introduction  to  Chemical  Preparations.     (Dunlap.) I2mo,  i   25 

Fletcher's  Practical  Instructions  in  Quantitative  Assaying  with  the  Blowpipe. 

i2mo,  morocco,  i  50 

Fowler's  Sewage  Works  Analyses i2mo,  2  oo 

Fresenius's  Manual  of  Qualitative  Chemical  Analysis.     (Wells.) 8vo,  5  oo 

Manual  of  Qualitative  Chemical  Analysis.  Part  I.  Descriptive.  (Wells.)  8vo,  3  oo 
System   of    Instruction    in    Quantitative    Chemical   Analysis.      (Cohn.) 

2  vols 8vo,  12  50 

Fuertes's  Water  and  Public  Health i2mo,  i  50 

Furman's  Manual  of  Practical  Assaying 8vo,  3  oo 

*  Getman's  Exercises  in  Physical  Chemistry i2mo,  2  oo 

Gill's  Gas  and  Fuel  Analysis  for  Engineers i2mo,  i  25 

*  Gooch  and  Browning's  Outlines  of  Qualitative  Chemical  Analysis.  Small  8vo,  i   25 

Grotenfelt's  Principles  of  Modern  Dairy  Practice.     (Woll.) i2mo,  2  oo 

Groth's  Introduction  to  Chemical  Crystallography  (Marshall) i2mo,  i  25 

Hammarsten's  Text-book  of  Physiological  Chemistry.     (Mandel.) 8vo,  4  oo 

Helm's  Principles  of  Mathematical  Chemistry.     (Morgan.) i2mo,  i  50 

Bering's  Ready  Reference  Tables  (Conversion  Factors) i6mo,  mcrocco,  2  50 

Hind's  Inorganic  Chemistry 8vo,  3  oo 

*  Laboratory  Manual  for  Students i2mo,  i  oo 

Holleman's  Text-book  of  Inorganic  Chemistry.     (Cooper.) 8vo,  2  50 

Text-book  of  Organic  Chemistry.     (Walker  and  Mott.) 8vo,  2  50 

*  Laboratory  Manual  of  Organic  Chemistry.     (Walker.) i2mo,  i  oo 

Hopkins's  Oil-chemists'  Handbook 8vo,  3  oo 

Iddings's  Rock  Minerals 8vo,  5  oo 

Jackson's  Directions  for  Laboratory  Work  in  Physiological  Chemistry.  .8vo,  i   25 

Keep's  Cast  Iron. 8vo,  2  50 

Ladd's  Manual  of  Quantitative  Chemical  Analysis I2mo,  i  oo 

Landauer's  Spectrum  Analysis.     (Tingle.) 8vo,  3  oo 

*  Langworthy  and  Austen.        The   Occurrence   of  Aluminium  in  Vegetable 

Products,  Animal  Products,  and  Natural  Waters 8vo,  2  oo 

Lassar-Cohn's  Application  of  Some  General  Reactions  to  Investigations  in 

Organic  Chemistry.  (Tingle.) i2mo,  i  oo 

Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Reference  to  State 

Control 8vo,  7  50 

Lob's  Electrochemistry  of  Organic  Compounds.  (Lorenz.) 8vo,  3  oo 

4 


Lodge's  Notes  on  Assaying  and  Metallurgical  Laboratory  Experiments.  ..  .8vo,  3  oo 

Low's  Technical  Method  of  Ore  Analysis ,  '. .  . . .".'.". 8vo,  3  oo 

Lunge's  Techno-chemical  Analysis.     (Cohn.) izmo  i  oo 

*  McKay  and  Larsen's  Principles  and  Practice  of  Butter-making 8vo,  i  50 

Mandel's  Handbook  for  Bio-chemical  Laboratory i2mo,  i  50 

*  Martin's  Laboratory  Guide  to  Qualitative  Analysis  with  the  Blowpipe .  .  i2mo,  60 
Mason's  Water-supply.     (Considered  Principally  from  a  Sanitary  Standpoint.) 

3d  Edition,  Rewritten 8vo,  4  oo 

Examination  of  Water.     (Chemical  and  Bacteriological.) I2mo,  i   25 

Matthew's  The  Textile  Fibres 8vo,  3  So 

Meyer's  Determination  of  Radicles  in  Carbon  Compounds.     (Tingle.).  .  I2mo, 

Miller's  Manual  of  Assaying i2mo, 

Cyanide  Process i2mo, 

Minet's  Production  of  Aluminum  and  its  Industrial  Use.     (Waldo.) .  .  .  .  I2mo, 

Mixter's  Elementary  Text-book  of  Chemistry I2mo, 

Morgan's  An  Outline  of  the  Theory  of  Solutions  and  its  Results i2mo, 

Elements  of  Physical  Chemistry i2mo,  3  co 

*  Physical  Chemistry  for  Electrical  Engineers..  . i2mo,  5  oo 

Morse's  Calculations  used  in  Cane-sugar  Factories i6mo,  morocco,  i  50 

*  Mu;r's  History  of  Chemical  Theories  and  Laws 8vo,  4  oo 

Mulliken's  General  Method  for  the  Identification  of  Pure  Organic  Compounds. 

Vol.  I Large  8vo,  5  oo 

O'Brine's  Laboratory  Guide  in  Chemical  Analysis 8vo,  2  oo 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

Ostwald's  Conversations  on  Chemistry.     Part  One.     (Ramsey.) i2mo,  i   50 

"                   "               "           "             Part  Two.     (Turnbull.) i2mo,  2  oo 

*  Pauli's  Physical  Chemistry  in  the  Service  of  Medicine.     (Fischer.) ....  i2mo,  i   25 

*  Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of  Mineral  Tests. 

8vo,  paper,  50 

Pictet's  The  Alkaloids  and  their  Chemical  Constitution.     (Biddle.) 8vo,  5  oo 

Pinner's  Introduction  to  Organic  Chemistry.     (Austen.) i2mo,  i   50 

Poole's  Calorific  Power  of  Fuels 8vo,  3  oo 

Prescott  and  Winslow's  Elements  of  Water  Bacteriology,  with  Special  Refer- 
ence to  Sanitary  Water  Analysis I2mo,  i  25 

*  Reisig's  Guide  to  Piece-dyeing 8vo,  25  oo 

Richards  and  Woodman's  Air,  Water,  and  Food  from  a  Sanitary  Standpoint..8vo,  2  oo 
Ricketts  and  Russell's  Skeleton  Notes  upon  Inorganic   Chemistry.     (Part  I. 

Non-metallic  Elements.) 8vo,  morocco,  75 

Ricketts  and  Miller's  Notes  on  Assaying 8vo,  3  oo 

Rideal's  Sewage  and  the  Bacterial  Purification  of  Sewage 8vo,  4  oo 

Disinfection  and  the  Preservation  of  Food 8vo,  4  oo 

Riggs's  Elementary  Manual  for  the  Chemical  Laboratory 8vo,  i  25 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc.) 8vo,  4  oo 

Ruddiman's  Incompatibilities  in  Prescriptions 8vo,  2  oo 

*  Whys  in  Pharmacy I2mo,  i  oo 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish 8vo,  3  oo 

Salkowski's  Physiological  and  Pathological  Chemistry.     (Ornddrff.) 8vo,  2  50 

Schimpf's  Text-book  of  Volumetric  Analysis i2mo,  2  50 

Essentials  of  Volumetric  Analysis i2mo,  i  25 

*  Qualitative  Chemical  Analysis 8vo,  i   25 

Smith's  Lecture  Notes  on  Chemistry  for  Dental  Students 8vo,  2  50 

Spencer's  Handbook  for  Chemists  of  Beet-sugar  Houses i6mo,  morocco,  3  oo 

Handbook  for  Cane  Sugar  Manufacturers i6mo,  morocco,  3  oo 

Stockbridge's  Rocks  and  Soils 8vo,  2  50 

*  Tillman's  Elementary  Lessons  in  Heat 8vo,  r  50 

*  Descriptive  General  Chemistry 8vo,  3  oo 

Treadwell's  Qualitative  Analysis.     (Hall.) 8vo,  3  oo 

Quantitative  Analysis.     (Hall.) 8vo,  4  oo 

Turneaure  and  Russell's  Public  Water-supplies 3vo,  5  oo 

5 


Van  Deventer's  Physical  Chemistry  for  Beginners.     (Boltwood.)  .  .      .  .  i2mo,  i  50 

*  Walke's  Lectures  on  Explosives  .................................  8vo,  4  oo 

Ware's  Beet-sugar  Manufacture  and  Refining  ..............  Small  8vo,  cloth  '  4  oo 

Washington's  Manual  of  the  Chemical  Analysis  of  Rocks  ...........      .8vo,  2  oo 

Weaver's  Military  Explosives  ...................................  .   8vo,'  3  oo 

Wehrenfennig's  Analysis  and  Softening  of  Boiler  Feed-Water  ..........  8vd,  4  oo 

Wells's  Laboratory  Guide  in  Qualitative  Chemical  Analysis  .............  8vo,  i  50 

Short  Course  in  Inorganic  Qualitative  Chemical  Analysis  for  Engineering 


I2mo>  T  SQ 

Text-book  of  Chemical  Arithmetic  ............................  i2mo,  i  25 

Whipple's  Microscopy  of  Drinking-water  ............................  8vo,  3  50 

Wilson's  Cyanide  Processes  ......................................  i2mo,  i  50 

Chlorination  Process  .......................................    i2mo,  i  50 

Winton's  Microscopy  of  Vegetable  Foods  ...........................  8vo,  7  50 

Wulling's    Elementary    Course    in  Inor^aLic,  Pharmaceutical,  and  Medical 

Chemistry  ..............................................  I2mo>  2  ^ 


CIVIL  ENGINEERING. 

BRIDGES    AND    ROOFS.       HYDRAULICS.       MATERIALS    OF    ENGINEERING. 
RAILWAY  ENGINEERING. 

Baker's  Engineers'  Surveying  Instruments i2mo,  3  oo 

Bixby's  Graphical  Computing  Table Paper  19^X24^  inches.  25 

Breed  and  Hosmer's  Pr'.nciples  and  Practice  of  Surveying 8vo,  3  oo 

*  Burr's  Ancient  and  Modern  Engineering  and  the  Isthmian  Canal   ....      8vo,  3  50 

Comstock's  Field  Astronomy  for  Engineers 8vo,  2  50 

Crandall's  Text-book  on  Geodesy  and  Least  Squares 8vo,  3  oo 

Davis's  Elevation  and  Stadia  Tables 8vo,  i  oo 

Elliott's  Engineering  for  Land  Drainage i2ir.o,  i  50 

Practical  Farm  Drainage 1200,  i  oo 

*Fiebeger's  Treatise  on  Civil  Engineering 8vo,  5  oo 

Flemer's  Phototopographic  Methods  and  Instruments 8vo,  5  oo 

Folwell's  Sewerage.     (Designing  and  Maintenance.) 8vo,  3  oo 

Freitag's  Architectural  Engineering.     2d  Edition,  Rewritten 8vo,  3  50 

French  and  Ives's  Stereotomy 8vo,  2  50 

Goodhue's  Municipal  Improvements i2mo,  i  75 

Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  oo 

Bering's  Ready  Reference  Tables  (Conversion  Factors') i6mo,  morocco,  2  50 

Howe's  Retaining  Walls  for  Earth i2mo,  i  25 

*  Ives's  Adjustments  of  the  Engineer's  Transit  and  Level i6mo,  Bds.  25 

Ives  and  Hilts's  Problems  in  Surveying i6mo,  morocco,  i  50 

Johnson's  (J.  B.)  Theory  and  Practice  of  Surveying Small  8vo,  4  oo 

Johnson's  (L.  J.)  Statics  by  Algebraic  and  Graphic  Methods 8vo,  2  oo 

Laplace's  Philosophical  Essay  on  Probabilities.     (Truscott  and  Emory.) .  i2mo,  2  oo 

Mahan's  Treatise  on  Civil  Engineering.     (1873.)     (Wood.) 8vo,  5  oo 

*  Descriptive  Geometry 8vo,  i  50 

Merriman's  Elements  of  Precise  Surveying  and  Geodesy. 8vo,  2  50 

Merriman  and  Brooks's  Handbook  for  Surveyors i6mo,  morocco,  2  oo 

Nugent's  Plane  Surveying 8vo,  3  50 

Ojden's  Sewer  Design I2mo,  2  oo 

Parsons's  Disposal  of  Municipal  Refuse 8vo,  2  oo 

Patton's  Treatise  on  Civil  Engineering 8vo  half  lealher,  7  50 

Reed's  Topographical  Drawing  and  Sketching 4to,  5  oo 

RideaPs  Sewage  and  the  Bacterial  Purification  of  Sewage .8vo,  4  oo 

Siebert  and  Biggin's  Modern  Stone-cutting  and  Masonry 8vo,  i  50 

6 


Smith's  Manual  of  Topographical  Drawing.     (McMillan.) 8vo,  2  50 

Sondericker's  Graphic  Statics,  with  Applications  to  \  russes,  Beams,  and  Arches. 

8vo,  2  oo 

Taylor  and  Thompson's  Treatise  on  Concrete,  Plain  and  Reinforced 8vo,  5  oo 

*  Trautwine's  Civil  Engineer's  Pocket-book i6mo,  morocco,  5  oo 

Venable's  Garbage  Crematories  in  America 8vo,  2  oo 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo  6  oo 

Sheep,  6  50 

Law  of  Operations  Preliminary  to  Construction  in  Engineering  and  Archi- 
tecture  8vo,  5  oo 

Sheep,  5  50 

Law  of  Contracts 8vo,  3  oo 

Warren's  Stereotomy — Problems  in  Stone-cutting 8vo,  2  50 

Webb's  Problems  in  the  Use  and  Adjustment  of  Engineering  Instruments. 

i6mo,  morocco,  i   25 

Wilson's  Topographic  Surveying 8vo,  3  50 


BRIDGES  AND  ROOFS. 

Boiler's  Practical  Treatise  on  the  Construction  of  Iron  Highway  Bridges.  .8vo,  2  oo 

*       Thames  River  Bridge 4to,  paper,  5  oo 

Burr's  Course  on  the  Stresses  In  Er.dgcs  and  Roof  Trusses,  A/ched  Ribs,  and 

Suspension  Bridges 8vo,  3  50 

Burr  and  Falk's  Influence  Lines  for  Bridge  and  Roof  Computations 8vo,  3  oo 

Design  and  Construction  of  Metall.c  Bridges 8vo  5  oo 

Du  Bois's  Mechanics  of  Engineer.ng.     Vol.  II Small  4to,  10  co 

Foster's  Treatise  on  Wooden  Trestle  Bridges 4to,  5  oo 

Fowler's  Ordinary  Foundations 8vo,  3  50 

Greene's  Roof  Trusses 8vo,  i  25 

Bridge  Trusses 8vo,  2  50 

Arches  in  Wood,  Iron,  and  Stone 8vo  2  50 

Howe's  Treatise  on  Arches 8vo,  4  oo 

Design  of  Simple  Roof-trusses  in  Wood  and  Steel » 8vo,  2  co 

Symmetrical  Masonry  Arches 8vo,  2  50 

Johnson,  Bryan,  and  Turneaure's  Theory  and  Practice  in  the  Designing  of 

Modern  Framed  Structures Small  410,  10  oo 

Merriman  and  Jacoby's  Text-book  on  Roofs  and  Bridges : 

Part  I.     Stresses  in  Simple  Trusses 8vo,  2  50 

Part  II.    Graphic  Statics 8vo,  2  50 

Part  III.  Bridge  Design 8vo,  2  50 

Part  IV.    Higher  Structures 8vo,  2  50 

Morison's  Memphis  Bridge 4to,  10  oo 

Waddell's  De  Pontibus,  a  Pocket-book  for  Bridge  Engineers.  . i6mo,  morocco,  2  oo 

*  Specifications  for  Steel  Bridges I2mo,  50 

Wright's  Designing  of  Draw-spans.     Two  parts  in  one  volume 8vo,  3  50 


HYDRAULICS. 

Barnes's  Ice  Formation 8vo,  3  oo 

Bazin's  Experiments  upon  the  Contraction  of  the  Liquid  Vein  Issuing  from 

an  Orifice.     (Trautwine.) 8vo,  2  oo 

Bovey's  Treatise  on  Hydraulics 8vo,  5  oo 

Church's  Mechanics  of  Engineering 8vo,  6  co 

Diagrams  of  Mean  Velocity  of  Water  in  Open  Channels paper,  i  5O 

Hydraulic  Motors 8vo,  2  oo 

Coffin's  Graphical  Solution  of  Hydrr.ulic  Problems i6mo,  morocco,  2  50 

Flather's  Dynamometers,  and  the  Measurement  of  Power i2mo,  3  oo 

7 


Folwell's  Water-supply  Engineering 8vc,  4  co 

Frizell's  Water-power. 8vo,  5  oo 

Fuertes's  Water  and  Public  Health i2mo,  i   50 

Water-filtration  Works. i2mo,  2  50 

Ganguillet  and  Kutter's  General  Formula  for  the  Uniform  Flow  of  Water  in 

Rivers  and  Other  Channels.     (Hering  and  Trautwine.) 8vo,  4  oo 

Hazen's  Filtration  of  Public  Water-supply 8vo,  3  oo 

Hazlehurst's  Towers  and  Tanks  for  Water- works 8vo,  2  50 

Herschel's  115  Experiments  on  the  Carrying  Capacity  of  Large,  Riveted,  Metal 

Conduits 8vo,  2  oo 

Mason's  Water-supply.     (Considered  Principally  from  a  Sanitary  Standpoint.) 

8vo,  4  oo 

Merriman's  Treatise  on  Hydraulics 8vo,  5  oo 

*  Michie's  Elements  of  Analytical  Mechanics 8vo,  4  oo 

Schuyler's   Reservoirs   for   Irrigation,   Water-power,   and    Domestic   Water- 
supply Large  8vo,  5  oo 

*  Thomas  and  Watt's  Improvement  of  Rivers 4to,  6  oo 

Turneaure  and  Russell's  Public  Water-supplies 8vo,  5  oo 

Wegmann's  Design  and  Construction  of  Dams 4to,  5  oo 

Water-supply  of  the  City  of  New  York  from  1658  to  1895 4to,  10  oo 

Whlpple's  Value  of  Pure  Water Large  i2mo,  i  oo 

Williams  and  Hazen's  Hydraulic  Tables 8vo,  i  50 

Wilson's  Irrigation  Engineering Smau  8vo,  4  oo 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  oo 

Wood's  Turbines 8vo,  2  50 

Elements  of  Analytical  Mechanics 8vo,  3  oo 


MATERIALS  OF  ENGINEERING. 

Baker's  Treatise  on  Masonry  Construction 8vo,  5  oo 

Roads  and  Pavements 8vo,  5  oo 

Black's  United  States  Public  Works Oblong  4to,  5  oo 

*  Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,  7  50 

Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering 8vo,  7  50 

Byrne's  Highway  Construction 8vo,  5  oo 

Inspection  of  the  Materials  and  Workmanship  Employed  in  Construction. 

i6mo,  3  oo 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

Du  Bois's  Mechanics -of  Engineering.     Vol.  I Small  4to,  7  50 

*Ecke,'s  Cements,  Limes,  and  Plasters 8vo,  6  oo 

Johnson's  Materials  of  Construction Large  8vo,  6  oo 

Fowler's  Ordinary  Foundations 8vo,  3  50 

Graves's  Forest  Mensuration f  vo,  4  co 

*  Greene's  Structural  Mechanics 8vo,  2  50 

Keep's  Cast  Iron 8vo,  2  50 

Lanza's  Applied  Mechanics 8vo,  7  50 

Marten's  Handbook  on  Testing  Materials.     (Henning.)     2  vols 8vo,  7  50 

Maurer's  Technical  Mechanics 8vo,  4  oo 

Merrill's  Stones  for  Building  and  Decoration 8vo,  5  oo 

Merriman's  Mechanics  of  Materials 8vo,  5  oo 

*  Strength  of  Materials I2mo,  i  oo 

Metcalf's  Steel.     A  Manual  for  Steel-users i2mo,  2  oo 

Patton's  Practical  Treatise  on  Foundations : 8vo,  5  oo 

Richardson's  Modern  Asphalt  Pavements 8vo,  3  oo 

Richey's  Handbook  for  Superintendents  of  Construction i6mo,  mor.,  4  oo 

*  Ries's  Clays:  Their  Occurrence,  Properties,  and  Uses 8vo,  5  oo 

Rockwell's  Roads  and  Pavements  in  France i2mo,  i  25 

8 


Sabin's  Industrial  and  Artistic  Technology  of  Paints  acd  Varnish 8vo,  3  oo 

Smith's  Materials  of  Machines .  .  . i2mo,  i  oo 

Snow's  Principal  Species  of  Wood 8vo,  3  50 

Spalding's  Hydraulic  Cement i2mo,  2  oo 

Text-book  on  Roads  and  Pavements 12 mo,  2  oo 

Taylor  and  Thompson's  Treatise  on  Concrete,  Plain  and  Reinforced 8vo,  5  oo 

Thurston's  Materials  of  Engineering.     3  Parts 8vo,  8  oo 

Part  I.     Non-metallic  Materials  of  Engineering  and  Metallurgy 8vo,  2  oo 

Part  II.     Iron  and  Steel 8vo,  3  50 

Part  III.     A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo,  2  50 

Tillson's  Street  Pavements  and  Paving  Materials 8vo,  4  oo 

Waddell's  De  Pontibus.    (A  Pocket-book  for  Bridge  Engineers.).  .  i6mo,  mor.,  2  oo 

*         Specifications  for  Steel  Bridges i2mo,  50 

Wood's  (De  V.)  Treatise  on  the  Resistance  of  Materials,  and  an  Appendix  on 

the  Preservation  of  Timber 8vo,  2  oo 

Wood's  (De  V.)  Elements  of  Analytical  Mechanics 8vo,  3  oo 

Wood's  (M.  P.)  Rustless  Coatings:    Corrosion  and  Electrolysis  of  Iron  and 

Steel 8vo,  4  oo 


RAILWAY  ENGINEERING. 

Andrew's  Handbook  for  Street  Railway  Engineers 3x5  inches,  morocco,  i  25 

Berg's  Buildings  and  Structures  of  American  Railroads 4to,  5  oo 

Brook's  Handbook  of  Street  Railroad  Location i6mo,  morocco,  i  50 

Butt's  Civil  Engineer's  Field-book i6mo,  morocco,  2  50 

Crandall's  Transition  Curve i6mo,  morocco,  i  50 

Railway  and  Other  Earthwork  Tables 8vo,  T   50 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book.  .  i6mo,  morocco,  5  oo 

Dredge's  History  of  the  Pennsylvania  Railroad:    (1879) Paper,  5  oo 

Fisher's  Table  of  Cubic  Yards Cardboard,  25 

Godwin's  Railroad  Engineers'  Field-book  and  Explorers'  Guide.  .  .  i6mo,  mor.,  2  50 
Hudson's  Tables  for  Calculating  the  Cubic  Contents  of  Excavations  and  Em- 
bankments  8vo,  i  oo 

Molitor  and  Beard's  Manual  for  Resident  Engineers 1 6mo,  i  oo 

Nagle's  Field  Manual  for  Railroad  Engineers i6mo,  morocco,  3  oo 

Philbrick's  Field  Manual  for  Engineers i6mo,  morocco,  3  oo 

Searles's  Field  Engineering i6mo,  morocco,  3  oo 

Railroad  Spiral i6mo,  morocco,  i  50 

Taylor's  Prismoidal  Formulae  and  Earthwork 8vo,  i  50 

*  Trautwine's  Method  of  Calculating  the  Cube  Contents  of  Excavations  and 

Embankments  by  the  Aid  of  Diagrams 8vo,  2  oo 

The  Field  Practice  of  Laying  Out  Circular  Curves  for  Railroads. 

i2mo,  morocco,  2  50 

Cross-section  Sheet Paper,  25 

Webb's  Railroad  Construction i6mo,  morocco,  5  oo 

Economics  of  Railroad  Construction Large  i2mo,  2  50 

Wellington's  Economic  Theory  of  the  Location  of  Railways Small  8vo,  5  oo 


DRAWING. 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing 8vo,  3  oo 

*  "  "        Abridged  Ed 8vo,  i  50 

Coolidge's  Manual  of  Drawing.      8vo,  paper,  i  oo 

9 


Coolidge  and  Freeman's  Elements  of  General  Drafting  for  Mechanical  Engi- 
neers  Oblong  4to,  2  50 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

Emch's  Introduction  to  Projective  Geometry  and  its  Applications 8vo,  2  50 

Hill's  Text-book  on  Shades  and  Shadows,  and  Perspective 8vo,  2  oo 

Jamison's  Elements  of  Mechanical  Drawing 8vo,  2  50 

Advanced  Mechanical  Drawing 8vo,  2  oo 

Jones's  Machine  Design: 

Part  I.     Kinematics  of  Machinery 8vo,  i  50 

Part  II.     Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

MacCord's  Elements  of  Descriptive  Geometry 8vo,  3  oo 

Kinematics;  or,  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams 8vo,  i  50 

MacLeod's  Descriptive  Geometry Small  8vo,  i  50 

*  Mahan's  Descriptive  Geometry  and  Stone-cutting 8vo,  i  50 

Industrial  Drawing.  (Thompson.) 8vo,  3  50 

Meyer's  Descriptive  Geometry 8vo,  2  oo 

Reed's  Topographical  Drawing  and  Sketching 4to,  5  oo 

Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design. 8vo,  3  oo 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Schwamb  and  Merrill's  Elements  of  Mechanism ..8vo,  3  oo 

Smith's  (R.  S.)  Manual  of  Topographical  Drawing.  (McMillan.) 8vo,  2  50 

Smith  (A.  W.)  and  Marx's  Machine  Design 8vo,  3  oo 

*  Titsworth's  Elements  of  Mechanical  Drawing Oblong  8vo,  25 

Warren's  Elements  of  Plane  and  Solid  Free-hand  Geometrical  Drawing.  i2mo,  oo 

Drafting  Instruments  and  Operations i2mo,  25 

Manual  of  Elementary  Projection  Drawing i2mo,  50 

Manual  of  Elementary  Problems  in  the  Linear  Perspective  of  Form  and 

Shadow i2mo,  oo 

Plane  Problems  in  Elementary  Geometry i2mo,  25 

Primary  Geometry. I2mo,  75 

Elements  of  Descriptive  Geometry,  Shadows,  and  Perspective 8vo,  3  50 

General  Problems  of  Shades  and  Shadows 8vo,  3  oo 

Elements  of  Machine  Construction  and  Drawing 8vo,  7  50 

Problems,  Theorems,  and  Examples  in  Descriptive  Geometry 8vo,  2  50 

Weisbach's    Kinematics    and    Power    of    Transmission.        (Hermann    and 

Klein.) 8vo,  5  oo 

Whelpley's  Practical  Instruction  in  the  Art  of  Letter  Engraving.  ......  12 mo,  2  oo 

Wilson's  (H.  M.)  Topographic  Surveying 8vo,  3  50 

Wilson's  (V.  T.)  Free-hand  Perspective 8vo.  2  50 

Wilson's  (V.  T.)  Free-hand  Lettering 8vo,  i  oo 

Woolf's  Elementary  Course  in  Descriptive  Geometry Large  8vo,  3  oo 


ELECTRICITY  AND  PHYSICS. 

*  Abegg's  Theory  of  Electrolytic  Dissociation.     (Von  Ende.) i2mo,  i  25 

Anthony  and  Brackett's  Text-book  of  Physics.     (Magie.) Small  8vo  3  oo 

Anthony's  Lecture-notes  on  the  Theory  of  Electrical  Measurements.  . .  .  i2mo,  i  oo 

Benjamin's  History  of  Electricity 8vo,  3  oo 

Voltaic  Cell 8vo,  3  oo 

Classen's  Quantitative  Chemical  Analysis  by  Electrolysis.     (Boltwood.).8vo,  3  oo 

*  Collins's  Manual  of  Wireless  Telegraphy i2mo,  i  50 

Morocco,  2  oo 

Crehore  and  Squier's  Polarizing  Photo-chronograph 8vo,  3  oo 

*  Danneel's  Electrochemistry.     (Merriam.) I2mo,  i  25 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book.  i6mo,  morocco,  5  oo 

10 


Dolezalek's    fheory    of    the    Lead   Accumulator    (Storage    Battery).      (Von 

Ende.) izmo,  2  50 

Duhem's  Thermodynamics  and  Chemistry.     (Burgess.) 8vo,  4  oo 

Flather's  Dynamometers,  and  the  Measurement  of  Power i2mo,  3  oo 

Gilbert's  De  Magnete.     (Mottelay.) 8vo,  2  50 

Hanchett's  Alternating  Currents  Explained i2mo,  i  oo 

Bering's  Ready  Reference  Tables  (Conversion  Factors) i6mo  morocco,  2  50 

Holman's  Precision  of  Measurements 8vo,  2  oo 

Telescopic   Mirror-scale  Method,  Adjustments,  and  Tests.  .  .  .Large  8vo,  75 

Kinzbrunner's  Testing  of  Continuous-current  Machines 8vo,  2  oo 

Landauer's  Spectrum  Analysis.     (Tingle.) 8vo,  3  oo 

Le  Chatelier's  High-temperature  Measurements.  (Boudouard — Burgess.)  i2mo,  3  oo 

Lob's  Electrochemistry  of  Organic  Compounds.     (Lorenz.) 8vo,  3  oo 

*  Lyons' 3  Treatise  on  Electromagnetic  Phenomena.   Vols.  I.  and  II.  8vo,  each,  6  oo 

*  Michie's  Elements  of  Wave  Motion  Relating  to  Sound  and  Light 8vo,  4  oo 

Niaudet's  Elementary  Treatise  on  Electric  Batteries.     (Fishback.) i2mo,  2  50 

*  Parshall  and  Hobart's  Electric  Machine  Design 4to,  half  morocco,  12  50 

Reagan's  Locomotives:    Simple,  Compound,  and  Electric.      New  Edition. 

Large  i2mo,  3  50 

*  Rosenberg's  Electrical  Engineering.     (Haldane  Gee — Kinzbrunner.).  .  .8vo,  2  oo 

Ryan,  Norris,  and  Hoxie's  Electrical  Machinery.     Vol.  1 8vo,  2  50 

Thurston's  Stationary  Steam-engines 8vo,  2  50 

*  Tillman's  Elementary  Lessons  in  Heat 8vo,  i  50 

Tory  and  Pitcher's  Manual  of  Laboratory  Physics Small  8vo,  2  oo 

Hike's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 


LAW. 

*  Davis's  Elements  of  Law 8vo,  2  50 

*  Treatise  on  the  Military  Law  of  United  States 8vo,  7  oo 

*  Sheep,  7  5<> 

*  Dudley's  Military  Law  and  the  Procedure  of  Courts-martial  .  .    .  Large  i2mo,  2  50 

Manual  for  Courts-martial i6mo,  morocco,  i  50 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  oo 

Sheep,  6  50 

Law  of  Operations  Preliminary  to  Construction  in  Engineering  and  Archi- 
tecture  8vo  5  oo 

Sheep,  5  5<> 

Law  of  Contracts 8vo,  3  oo 

Winthrop's  Abridgment  of  Military  Law I2mo,  a  50 


MANUFACTURES. 

Bernadou'S  Smokeless  Powder — Nitro-cellulose  and  Theory  of  the  Cellulose 

Molecule i2mo,  2  50 

Bolland's  Iron  Founder i2mo,  2  50 

The  Iron  Founder,"  Supplement I2mo,  2  50 

Encyclopedia  of  Founding  and  Dictionary  of  Foundry  Terms  Used  in  the 

Practice  of  Moulding i2mo,  3  oo 

*  Claassen's  Beet-sugar  Manufacture.    (Hall  and  Rolfe.) 8vo,  3  oo 

*  Eckel's  Cements,  Limes,  and  Plasters 8vo,  6  oo 

Eissler's  Modern  High  Explosives 8vo,  4  oo 

Effront's  Enzymes  and  their  Applications.     (Prescott.) 8vo,  3  oo 

Fitzgeralc'.'s  Boston  Machinist .- I2mo,  i  oo 

Ford's  Boiler  Making  for  Boiler  Makers i8mo,  i  oo 

Hopkin's  Oil-chemists'  Handbook 8vo,  3  oo 

Keep's  Cast  Iron 8vo,  2  50 

11 


Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Reference  to  State 

Control. Large  8vo,  7  50 

*  McKay  and  Larsen's  Principles  and  Practice  of  Butter-making 8vo,  i  50 

Matthews's  The  Textile  Fibres 8vo,  3  50 

Metcalf's  Steel.     A  Manual  for  Steel-users: i2mo,  2  oo 

MetcalfeV  Cost  of  Manufactures — And  the  Administration  of  Workshops. 8vo,  5  oo 

Meyer's  Modern  Locomotive  Construction 4to,  10  oo 

Morse's  Calculations  used  in  Cane-sugar  Factories i6mo,  morocco,  i  50 

*  Reisig's  Guide  to  Piece-dyeing. 8vo,  25  oo 

Rice's  Concrete-block  Manufacture 8vo,  2  oo 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish 8vo,  3  oo 

Smith's  Press-working  of  Metals 8vo,  3  oo 

Spalding's  Hydraulic  Cement i2mo,  2  oo 

Spencer's  Handbook  for  Chemists  of  Beet-sugar  Houses i6mo  morocco,  3  oo 

Handbook  for  Cane  Sugar  Manufacturers i6mo  morocco,  3  oo 

Taylor  and  Thompson's  Treatise  on  Concrete,  Plain  and  Reinforced 8vo,  5  oo 

Thurston's  Manual  of  Steam-boilers,  their  Designs,  Construction  and  Opera- 
tion  8vo,  5  oo 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

Ware's  Beet-sugar  Manufacture  and  Refining • Small  8vo,  4  oo 

Weaver's  Mil'.tary  Explosives 8vo,  3  oo 

West's  American  Foundry  Practice i2mo,  2  50 

Moulder's  Text-book i2mo,  2  50 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  oo 

Wood's  Rustless  Coatings :    Corrosion  and  Electrolysis  of  Iron  and  Steel.  .8vo,  4  oo 


MATHEMATICS. 

Baker's  Elliptic  Functions. 8vo,  i  50 

*  Bass's  Elements  of  Differential  Calculus, i2mo,  4  oo 

Briggs's  Elements  of  Plane  Analytic  Geometry I2mo,  oo 

Compton's  Manual  of  Logarithmic  Computations i2mo  50 

Davis's  Introduction  to  the  Logic  of  Algebra 8vo,  50 

*  Dickson's  College  Algebra Large  i2mos  50 

*  Introduction  to  the  Theory  of  Algebraic  Equations Large  i2mo,  25 

Emch's  Introduction  to  Projective  Geometry  and  its  Applications 8vo  50 

Halsted's  Elements  of  Geometry 8vo,  75 

Elementary  Synthetic  Geometry, 8vo,  50 

Rational  Geometry I2mo,  75 

*  Johnson's  (J.  B.)  Thrte-place  Logarithmic  Tables:   Vest-pocket  size. paper,  15 

100  copies  for  5  oo 

*  Mounted  on  heavy  cardboard,  8X 10  inches,  25 

10  copies  for  2  oo 

Johnson's  (W.  W.)  Elementary  Treatise  on  Differential  Calculus .  .  Small  8vo,  3  oo 

Elementary  Treatise  on  the  Integral  Calculus SmalfSvo,  i  50 

Johnson's  (W.  W.)  Curve  Tracing  in  Cartesian  Co-ordinates, i2mo,  i  oo 

Johnson's  (W.  W.)  Treatise  on  Ordinary  and  Partiaf  Differential  Equations. 

Small  8vo,  3  50 

Johnson's  (W,  W.)  Theory  of  Errors  and  the  Method  of  Least  Squares.  i2mo,  i  50 

*  Johnson's  (W.  W.)  Theoretical  Mechanics i2mo,  3  oo 

Laplace's  Philosophical  Essay  on  Probabilities.    (Truscott  and  Emory.) .  i2mo,  2  oo 

*  Ludlow  and  Bass.     Elements  of  Trigonometry  and  Logarithmic  and  Other 

Tables 8vo,  3  oo 

Trigonometry  and  Tables  published  separately Each,  2  oc 

*  Ludlow's  Logarithmic  and  Trigonometric  Tables 8vo  i  oo 

Manning's  Irrational  Numbers  and  their  Representation  by  Sequences  and  Series 

i2mo,  i  25 
12 


Mathematical  Monographs.     Edited  by  Mansfield  Merriman  and  Robert 

S.  Woodward Octavo,  each     i  oo 

No.  i.  History  of  Modern  Mathematics,  by  David  Eugene  Smith. 
No.  2.  Synthetic  Projective  Geometry,  by  George  Bruce  Halsted. 
No.  3.  Determinants,  by  Laenas  Gifford  Weld.  No.  4.  Hyper- 
bolic Functions,  by  James  McMahon.  No.  5.  Harmonic  Func- 
tions, by  William  E.  Byerly.  No.  6.  Grassmann's  Space  Analysis, 
by  Edward  W.  Hyde.  No.  7.  Probability  and  Theory  of  Errors, 
by  Robert  S.  Woodward.  No.  8.  Vector  Analysis  and  Quaternions, 
by  Alexander  Macfarlane.  No.  9.  Differential  Equations,  by 
William  Woolsey  Johnson.  No.  10.  The  Solution  of  Equations, 
by  Mansfield  Merriman.  No.  n.  Functions  of  a  Complex  Variable, 
by  Thomas  S.  Fiske. 

Maurer's  Technical  Mechanics 8vo,    4  oo 

Merriman's  Method  of  Least  Squares 8vo,     2  oo 

Rice  and  Johnson's  Elementary  Treatise  on  the  Differential  Calculus. .  Sm.  8vo,     3  oo 

Differential  and  Integral  Calculus.     2  vols.  in  one Small  8vo,    2  50 

*  Veblen  and  Lennes's  Introduction  to  the  Real  Infinitesimal  Analysis  of  One 

Variable 8vo,    2  oo 

Wood's  Elements  of  Co-ordinate  Geometry 8vo,    2  oo 

Trigonometry:   Analytical,  Plane,  and  Spherical i2mo,    i  oo 


MECHANICAL  ENGINEERING. 

MATERIALS  OF  ENGINEERING,  STEAM-ENGINES  AND  BOILERS. 

Bacon's  Forge  Practice i2mo,  i  50 

Baldwin's  Steam  Heating  for  Buildings I2mo,  2  50 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing 8vo,  3  oo 

*  "                   "                 "        Abridged  Ed 8vo,  i  50 

Benjamin's  Wrinkles  and  Recipes I2mo,  2  oo 

Carpenter's  Experimental  Engineering * 8vo,  6  oo 

Heating  and  Ventilating  Buildings 8vo,  4  oo 

Clerk's  Gas  and  Oil  Engine Small  8vo,  4  oo 

Coolidge's  Manual  of  Drawing 8vo,  paper,  i  oo 

Coolidge  and  Freeman's  Elements  of  General  Drafting  for  Mechanical  En- 
gineers  Oblong  4to,  2  50 

Cromwell's  Treatise  on  Toothed  Gearing i2mo,  i  50 

Treatise  on  Belts  and  Pulleys I2mo,  i  50 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

Flather's  Dynamometers  and  the  Measurement  of  Power. i2mo,  3  oo 

Rope  Driving I2mo,  2  oo 

Gill's  Gas  and  Fuel  Analysis  for  Engineers i2mo,  i  25 

Hall's  Car  Lubrication i2mo,  i  oo 

Bering's  Ready  Reference  Tables  (Conversion  Factors) i6mo,  morocco,  2  50 

Button's  The  Gas  Engine. 8vo,  5  r.o 

Jamison's  Mechanical  Drawing 8vo,  2  50 

Jones's  Machine  Design: 

Part  I.     Kinematics  of  Machinery 8vo,  i  50 

Part  II.     Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

Kent's  Mechanical  Engineers'  Pocket-book i6mo,  morocco,  5  oo 

Kerr's  Power  and  Power  Transmission 8vo,  2  oo 

Leonard's  Machine  Shop,  Tools,  and  Methods 8vo,  4  oo 

*  Lorenz's  Modern  Refrigerating  Machinery.    (Pope,  Baven,  and  Dean.)  .  .  8vo,  4  oo 
MacCord's  Kinematics;   cr,  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams 8vo,  i  50 

13 


MacFar land's  Standard  Reduction  Factors  for  Gases 8vo,  i  50 

Ilahan's  Industrial  Drawing.     (Thompson.) 8vo  3  50 

Pooie's  Calorific  Power  of  Fuels 8vo,  3  oo 

Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design. 8vo,  3  oo 

Richard's  Compressed  Air i2mo,  i  50 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Schwaob  and  Merrill's  Elements  of  Mechanism 8vo,  3  oo 

Smith's  (O.)  Press-working  of  Metals 8vo  3  oo 

Smith  (A.  W.)  and  Marx's  Machine  Design 8vo,  3  oo 

Thurston's   Treatise   on   Friction  and   Lost   Work   in   Machinery   and   Mill 

Work 8vo8  3  oo 

Animal  as  a  Machine  and  Prime  Motor,  and  the  Laws  of  Energetics .  12010,  i  oo 

Tillson's  Complete  Automobile  Instructor i6mo,  i  50 

Morocco,  2  oo 

Warren's  Elements  of  Machine  Construction  and  Drawing 8vo,  7  50 

Weisbach's    Kinematics    and    the    Power    of    Transmission.     (Herrmann — 

Klein.) 8vo,  5  oo 

Machinery  of  Transmission  and  Governors.     (Herrmann — Klein.).  .8vo,  5  oo 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  oo 

Wood's  Turbines 8vo,  2  50 


MATERIALS  OF  ENGINEERING. 

*  Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,  7  50 

Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering.    6th  Edition. 

Reset 8vo,  7  50 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

*  Greene's  Structural  Mechanics 3vo,  2  50 

Johnson's  Materials  of  Construction 8vo,  6  oo 

Keep's  Cast  Iron 8vo,  2  50 

Lanza's  Applied  Mechanics 8vo,  7  50 

Martens 's  Handbook  on  Testing  Materials.     (Henning.) 8vo,  7  50 

Maurer's  Technical  Mechanics 8vo,  4  oo 

Merriman's  Mechanics  of  Materials 8vo,  5  oo 

*  Strength  of  Materials i2mo,  i  oo 

Metcalf's  Steel.     A  Manual  for  Steel-users i2mo,  2  oo 

Sabin's  Industrial  and  Artistic  Technology  of  Paints  and  Varnish 8vo,  3  oo 

Smith's  Materials  of  Machines I2mo,  i  oo 

Thurston's  Materials  of  Engineering 3  vols.,  8vo,  8  oo 

Part  II.     Iron  and  Steel 8vo,  3  50 

Part  III.     A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo,  2  50 

Wood's  (De  V.)  Treatise  on  the  Resistance  of  Materials  and  an  Appendix  on 

the  Preservation  of  Timber 8vo,  2  oo 

Elements  of  Analytical  Mechanics 8vo,  3  oo 

Wood's  (M.  P.)  Rustless  Coatings:    Corrosion  and  Electrolysis  of  Iron  and 

Steel 8vo,  4  oo 


STEAM-ENGINES  AND  BOILERS. 

Berry's  Temperature-entropy  Diagram I2mo,  i   25 

Carnot's  Reflections  on  the  Motive  Power  of  Heat.     (Thurston.) i2mo,  i   50 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book.  .  .  .i6mo,  mor.,  5  oo 

Ford's  Boiler  Making  for  Boiler  Makers i8mo,  i  oo 

Goss's  Locomotive  Sparks 8vo,  2  oo 

Locomotive  Performance 8vo,  5  oo 

Hemenway's  Indicator  Practice  and  Steam-engine  Economy I2mo,  2  oo 

14 


Button's  Mechanical  Engineering  of  Power  Plants 8vo,  5  oo 

Heat  and  Heat-engines 8vo.  5  oo 

Kent's  Steam  boiler  Economy 8vo,  4  oo 

Kneass's  Practice  and  Theory  of  the  Injector 8vo,  i  50 

MacCord's  Slide-valves 8vo,  2  oo 

Meyer's  Modern  Locomotive  Construction 4to,  10  oc 

Peabody's  Manual  of  the  Steam-engine  Indicator I2mo,  *  50 

Tables  of  the  Properties  of  Saturated  Steam  and  Other  Vapors    8vo,  i  oo 

Thermodynamirs  of  the  Steam-engine  and  Other  Heat-engines 8vo,  5  oo 

Valve-gears  for  Steam-engines 8vo,  2  50 

Peabody  and  Miller's  Steam-boilers 8vo,  4  oo 

Pray's  Twenty  Years  with  the  Indicator Large  8vo,  2  50 

Pupin's  Thermodynamics  of  Reversible  Cycles  in  Gases  and  Saturated  Vapors. 

(Osterberg.) I2mo,  i  25 

Reagan's  Locomotives:    Simple,  Compound,  and  Electric.     New  Edition. 

Large  12010,  3  50 

Rontgen's  Principles  of  Thermodynamics.     (Du  Bois.) 8vo,  5  off 

Sinclair's  Locomotive  Engine  Running  and  Management 12 mo,  2  oo 

Smart's  Handbook  of  Engineering  Laboratory  Practice i2mo,  2  50 

Snow's  Steam-boiler  Practice 8vo,  3  oo 

Spangler's  Valve-gears 8vo,  2  50 

Notes  on  Thermodynamics i2mo,  i  oo 

Spangler,  Greene,  and  Marshall's  Elements  of  Steam-engineering 8vo,  3  oo 

Thomas's  Steam-turbines 8vo,  3  50 

Thurston's  Handy  Tables 8vo,  i  50 

Manual  of  the  Steam-engine 2  vols.,  8vo,  10  oo 

Part  I.     History,  Structure,  and  Theory 8vo,  6  oo 

Part  II.     Design,  Construction,  and  Operation 8vo,  6  oo 

Handbook  of  Engine  and  Boiler  Trials,  and  the  Use  of  the  Indicator  and 

the  Prony  Brake 8vo,  5  oo 

Stationary  Steam-engines 8vo,  2  50 

Steam-boiler  Explosions  in  Theory  and  in  Practice I2mo,    I  50 

Manual  of  Stpam-boilers,  their  Designs,  Construction,  and  Operation  .8vo,  5  oo 

Wehrenfenning's  Analysis  and  Softening  of  Boiler  Feed-water  (Patterson)  8vo,  4  oo 

Weisbach's  Heat,  Steam,  and  Steam-engines.     (Du  Bois.) 8vo,  5  oo 

Whitham's  Steam-engine  Design 8vo,  5  oo 

Wood's  Thermodynamics,  Heat  Motors,  and  Refrigerating  Machines.  .  ,8vo,  4  oo 


MECHANICS  AND   MACHINERY. 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bovey's  Strength  of  Materials  and  Theory  of  Structures   8vo,  7  50 

Chase's  The  Art  of  Pattern-making i2mo,  2  50 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

Notes  and  Examples  in  Mechanics 8vo,  2  oo 

Compton's  First  Lessons  in  Metal-working I2mo,  i  50 

Compton  and  De  Groodt's  The  Speed  Lathe i2mo,  i   «;o 

Cromwell's  Treatise  on  Toothed  Gearing i2mo,  i   50 

Treatise  on  Belts  and  Pulleys i2mo,  i  50 

Dana's  Text-book  of  Elementary  Mechanics  for  Colleges  and  Schools.  .i2mo,  i  50 

Dingey's  Machinery  Pattern  Making i2mo,  2  oo 

Dredge's   Record  of   the   Transportation  Exhibits   Building  of  the   World's 

Columbian  Exposition  of  1893 4to  half  morocco,  5  oo 

Du  Bois's  Elementary  Principles  of  Mechanics: 

Vol.      I.     Kinematics 8vo,  3  50 

Vol.    II.     Statics 8vo.  4  oo 

Mechanics  of  Engineering.     Vol.    I Small  4to,  7  50 

Vol.  II Small  4to,  10  oo 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

15 


Fitzgerald's  Boston  Machinist i6mo,  i  oo 

Flather's  Dynamometers,  and  the  Measurement  of  Power i2mo,  3  oo 

Rope  Driving i2mo,  2  oo 

Goss's  Locomotive  Sparks 8vo,  2  oo 

Locomotive  Performance 8vo,  5  oo 

*  Greene's  Structural  Mechanics. .  .    8vo,  2  50 

Hall's  Car  Lubrication i2mo,  i  oo 

Holly's  Art  of  Saw  Filing i8mo,         75 

James's  Kinematics  of  a  Point  and  the  Rational  Mechanics  of  a  Particle. 

Small  8vo,  2  oo 

*  Johnson's  (W.  W.)  Theoretical  Mechanics I2mo,  3  oo 

Johnson's  (L.  J.)  Statics  by  Graphic  and  Algebraic  Methods 8vo,  2  oo 

Jones's  Machine  Design: 

Part    I.     Kinematics  of  Machinery 8vo,  i  50 

Part  II.     Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

Kerr's  Power  and  Power  Transmission 8vo,  2  oo 

Lanza's  Applied  Mechanics 8vo,  7  50 

Leonard's  Machine  Shop,  Tools,  and  Methods 8vo,  4  oo 

*  Lorenz's  Modern  Refrigerating  Machinery.     (Pope,  Haven,  and  Dean.). 8vo,  4  oo 
MacCord's  Kinematics;  or,  Practical  Mechanism 8vo,  5  oo 

Velocity  Diagrams 8vo,  i   50 

*  Martin's  Text  Book  on  Mechanics,  Vol.  I,  Statics 121110,  i  25 

Maurer's  Technical  Mechanics 8vo,  4  oo 

Merriman's  Mechanics  of  Materials 8vo,  5  oo 

*  Elements  of  Mechanics I2mo,  i  oo 

*  Michie's  Elements  of  Analytical  Mechanics 8vo,  4  oo 

*  Parshalland  Hobart's  Electric  Machine  Design 4to,  half  morocco,  12  50 

Reagan's  Locomotives :  Simple,  Compound,  and  Electric.     New  Edition. 

Large  i2tno,  3  oo 

Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design. 8vo,  3  oo 

Richards's  Compressed  Air '. i2mo,  i  50 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Ryan,  Norris,  and  Hoxie's  Electrical  Machinery.     Vol.  1 8vo,  2  50 

Sanborn's  Mechanics:  Problems Large  12010,  i   50 

Schwamb  and  Merrill's  Elements  of  Mechanism 8vo,  3  oo 

Sinclair's  Locomotive-engine  Running  and  Management I2mo,  2  oo 

Smith's  (O.)  Press-working  of  Metals 8vo,  3  oo 

Smith's  (A.  W.)  Materials  of  Machines i2mo,  i  oo 

Smith  (A.  W.)  and  Marx's  Machine  Design 8vo,  3  oo 

Spangler,  Greene,  and  Marshall's  Elements  of  Steam-engineering 8vo,  3  oo 

Thurston's   Treatise  on  Friction   and  Lost  Work  in    Machinery  and    Mill 

Work 8vo,  3  oo 

Animal  as  a  Machine  and  Prime  Motor,  and  the  Laws  of  Energetics.  1 2mo,  i  oo 

Tillson's  Complete  Automobile  Instructor i6mo,  i  50 

Morocco,  2  oo 

Warren's  Elements  of  Machine  Construction  and  Drawing 8vo,  7  50 

Weisbach's  Kinematics  and  Power  of  Transmission.    (Herrmann — Klein.).  8vo,  5  oo 

Machinery  of  Transmission  and  Governors.      (Herrmann — Klein.). 8vo,  5  oo 

Wood's  Elements  of  Analytical  Mechanics 8vo,  3  oo 

Principles  of  Elementary  Mechanics I2mo,  i  25 

Turbines 8vo,  2  50 

The  World's  Columbian  Exposition  of  1893 4to,  i  oo 

MEDICAL. 

De  Fursac's  Manual  of  Psychiatry.     (Rosanoff  and  Collins.) Large  i2roo,  2  50 

Ehrlich's  Collected  Studies  on  Immunity.     (Bolduan.) 8vo,  6  oo 

Hammarsten's  Text-book  on  Physiological  Chemistry.     (Mandel.) 8vo,  4  oo 

16 


Lassar-Cohn's  Practical  Urinary  Analysis.     (Lorenz.) i2mo,  i  oo 

*  Pauli's  Physical  Chemistry  in  the  Service  of  Medicine.     (Fischer.) ...    i2mo,  i  25 

*  Pozzi-Escot's  The  Toxins  and  Venoms  and  their  Antibodies.     (Cohn.).  iimo,  i  oo 

Rostoski's  Serum  Diagnosis.     (Bolduan.) • i2mo,  i  oo 

Salkowski's  Physiological  and  Pathological  Chemistry.     (Crndorff.) 8vo,  2  50 

*  Satterlee's  Outlines  of  Human  Embryology i2mo,  i   25 

Steel's  Treatise  on  the  Diseases  of  the  Dog 8vo,  3  50 

Von  Behring's  Suppressfon  of  Tuberculosis.     (Bolduan.) i2mo,  i  oo 

Wassermann's  Immune  Sera  •  Haemolysis,  Cytotoxins,  and  Precipitins.     (Bol- 
duan.)   i2mo,  cloth,  i  oo 

Woodhull's  Notes  on  Military  Hygiene i6mo,  i  50 

*  Personal  Hygiene I2mo,  i  oo 

Wulling's  An  Elementary  Course  in  Inorganic  Pharmaceutical  and  Medical 

Chemistry i2mo,  2  oo 


METALLURGY. 

Egleston's  Metallurgy  of  Silver,  Gold,  and  Mercury: 

Vol.    I.     Silver 8vo,  7  50 

Vol.  II.     Gold  and  Mercury 8vo,  7  50 

Goesel's  Minerals  and  Metals:     A  Reference  Book , . . .  .  i6mo,  mor.  3  oo 

*  Iles's  Lead-smelting i2mo,  2  50 

Keep's  Cast  Iron 8vo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Europe 8vo,  i   50 

Le  Chatelier's  High-temperature  Measurements.  (Boudouard — Burgess. )i2mo,  3  oo 

Metcalf's  Steel.     A  Manual  for  Steel-users i2mo,  2  oo 

Miller's  Cyanide  Process i2mo,  i  oo 

Minet's  Production  of  Aluminum  and  its  Industrial  Use.     (Waldo.). .    .  i2mo,  2  50 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc.) 8vo,  4  oo 

Smith's  Materials  of  Machines i2mo,  i  oo 

Thurston's  Materials  of  Engineering.     In  Three  Parts 8vo,  8  oo 

Part    II.     Iron  and  Steel 8vo,  3  50 

Part  III.     A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo,  2  50 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 


MINERALOGY. 

Barringer's  Description  of  Minerals  of  Commercial  Value.    Oblong,  morocco,  2  50 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  oo 

Map  of  Southwest  Virignia Pocket-book  form.  2  oo 

Brush's  Manual  of  Determinative  Mineralogy.     (Penfield.) 8vo,  4  oo 

Chester's  Catalogue  of  Minerals.  ».  . 8vo,  paper,  i  oo 

Cloth,  i  25 

Dictionary  of  the  Names  of  Minerals 8vc,  3  50 

Dana's  System  of  Mineralogy Large  8vo,  half  leather,  12  50 

First  Appendix  to  Dana's  New  "  System  of  Mineralogy." Large  8vo,  i  oo 

Text-book  of  Mineralogy 8vo,  4  oo 

Minerals  and  How  to  Study  Them I2mo.  i  50 

Catalogue  of  American  Localities  of  Minerals Large  8vo,  i  oo 

Manual  of  Mineralogy  and  Petrography I2mo  2  oo 

Douglas's  Untechnical  Addresses  on  Technical  Subjects i2mo,  i  oo 

Eakle's  Mineral  Tables 8vo,  i  25 

Egleston's  Catalogue  of  Minerals  and  Synonyms 8vo,  2  50 

Goesel's  Minerals  and  Metals :     A  Reference  Book i6mo,  mor.  3  oo 

Groth's  Introduction  to  Chemical  Crystallography  (Marshall) i2mo,  i  25 

17 


Iddings's  Rock  Minerals 8vo,  5  oo 

Merrill's  Non-metallic  Minerals:   Their  Occurrence  and  Uses 8vo,  4  oo 

*  Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of  Mineral  Tests. 

8vo,  paper,  50 

*  Richards's  Synopsis  of  Mineral  Characters i2mo,  morocco,  i  25 

*  Ries's  Clays:  Their  Occurrence,  Properties,  and  Uses 8vo,  5  oo 

Rosenbusch's   Microscopical   Physiography   of   the    Rock-making  Minerals. 

(Iddings.) 8vo,  5  oo 

*  Tollman's  Text-book  of  Important  Minerals  and  Rocks 8vo,  2  oo 


MINING. 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  oo 

Map  of  Southwest  Virginia Pocket-book  form  2  oo 

Douglas's  Untechnical  Addresses  on  Technical  Subjects I2mo,  I  oo 

Eissler's  Modern  High  Explosives c 8  4  no 

Goesel's  Minerals  and  Metals :     A  Reference  Book .  .      i6mo,  mor.  3  oo 

Goodyear's  Coal-mines  of  the  Western  Coast  of  the  United  States i2mo,  2  50 

Ihlseng's  Manual  of  Mining 8vo,  5  oo 

*  Iles's  Lead-smelting I2mo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Europe 8vo,  i  50 

Miller's  Cyanide  Process i2mo,  i  oo 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

Robine  and  Lenglen's  Cyanide  Industry.     (Le  Clerc.) 8vo,  4  oo 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

Weaver's  Military  Explosives 8vo,  3  oo 

Wilson's  Cyanide  Processes i2mo,  I  50 

Chlorination  Process I2mo,  i  50 

Hydraulic  and  Placer  Mining. i2mo,  2  oo 

Treatise  on  Practical  and  Theoretical  Mine  Ventilation i2ino,  i  25 


SANITARY  SCIENCE. 

Bashore's  Sanitation  of  a  Country  House 1 2  mo ,  i  oo 

*  Outlines  of  Practical  Sanitation i2mo,  i  25 

Folwe.U's  Sewerage.     (Designing,  Construction,  and  Maintenance.) 8vo,  3  oo 

Water-supply  Engineering 8vo,  4  oo 

Fowler's  Sewage  Works  Analyses i2ma,  2  oo 

Fuertes's  Water  and  Public  Health i2mo,  i  50 

Water-filtration  Works i2mo,  2  50 

Gerhard's  Guide  to  Sanitary  House-inspection i6mo,  i  oo 

Hazen's  Filtration  of  Public  Water-supplies 8vo,  3  oo 

Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Reference  to  State 

Control 8vo,  7  50 

Mason's  Water-supply.  ( Considered  pr inc  ipally  from  a  Sanitary  Standpoint)  8vo ,  4  oo 

Examination  of  Water.     (Chemical  and  Bacteriological.) I2mo,  i   25 

*  Merriman's  Elements  of  Sanitary  Engineering 8vo,  2  oo 

Ogden's  Sewer  Design i2mo,  2  oo 

Prescott  and  Winslow's  Elements  of  Water  Bacteriology,  with  Special  Refer- 


ence to  Sanitary  Water  Analysis i2mo, 

*  Price's  Handbook  on  Sanitation 12010, 

Richards's  Cost  of  Food.     A  Study  in  Dietaries i2mo, 

Cost  of  Living  as  Modified  by  Sanitary  Science i2mo, 

Cost  of  Shelter i2mo, 

18 


Richards  and  Woodman's  Air.  Water,  and  Food  from  a  Sanitary  Stand- 
point  8vo,  2  oo 

*  Richards  and  Williams's  The  Dietary  Computer 8vo  i   50 

Rideal's  Sewage  and  Bacterial  Purification  of  Sewage 8vo,  4  oo 

Turneaure  and  Russell's  Public  Water-supplies 8vo,  5  oo 

Von  Behring's  Suppression  of  Tuberculosis.     (Bolduan.) i2mo,  i  oo 

Whipple's  Microscopy  of  Drinking-water 8vo,  3  50 

Winton's  Microscopy  of  Vegetable  Foods 8vo,  7  50 

Woodhull's  Notes  on  Military  Hygiene i6mo,  i  50 

*  Personal  H/giene, xarno,  i  oo 


MISCELLANEOUS. 

Emmons's  Geological  Guide-book  of  the  Rocky  Mountain  Excursion  of  the 

International  Congress  of  Geologists Large  8vo,  i  50 

FerreFs  Popular  Treatise  en  the  Winds 8vo,  4  oo 

Gannett's  Statistical  Abstract  of  the  World   24010,  75 

Haines's  American  Railway  Management 12010,  2  50 

Ricketts's  History  of  Rensselaer  Polytechnic  Institute.  1824-1894. .Small  8vo,  3  oo 

Rotherham's  Emphasized  New  Testament Large  8vo ,  2  oo 

The  World's  Columbian  Ixposition  of  1893 4to,  i  oo 

Winslow's  Elements  of  Applied  Microscopy 12010,  i  50 


HEBREW  AND  CHALDEE  TEXT-BOOKS. 

Green's  Elementary  Hebrew  Grammar 12010,  i  25 

Hebrew  Chrestomathy 8vo,  2  oo 

Gesenius's  Hebrew  and  Chaldee  Lexicon  to  the  Old  Testament  Scriptures. 

(Tregelles.).  , Small  4to,  half  morocco,  5  oo 

Letteris's  Hebrew  Bible 8vo,  2  25 

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U.C.  BERKELEY  LIBRARIES 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


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